diff --git a/core/src/main/scala/org/apache/spark/ui/UIUtils.scala b/core/src/main/scala/org/apache/spark/ui/UIUtils.scala
index 3b2b944da725b..0f1ea20413db6 100644
--- a/core/src/main/scala/org/apache/spark/ui/UIUtils.scala
+++ b/core/src/main/scala/org/apache/spark/ui/UIUtils.scala
@@ -358,7 +358,7 @@ private[spark] object UIUtils extends Logging {
       <div id="dag-viz-metadata">
         {
           graphs.map { g =>
-            <div class="stage-metadata" stageId={g.rootScope.id} style="display:none">
+            <div class="stage-metadata" stageId={g.rootCluster.id} style="display:none">
               <div class="dot-file">{VizGraph.makeDotFile(g, forJob)}</div>
               { g.incomingEdges.map { e => <div class="incoming-edge">{e.fromId},{e.toId}</div> } }
               { g.outgoingEdges.map { e => <div class="outgoing-edge">{e.fromId},{e.toId}</div> } }
diff --git a/core/src/main/scala/org/apache/spark/ui/viz/VisualizationListener.scala b/core/src/main/scala/org/apache/spark/ui/viz/VisualizationListener.scala
index e6ea15787b907..95a6cb729e1d1 100644
--- a/core/src/main/scala/org/apache/spark/ui/viz/VisualizationListener.scala
+++ b/core/src/main/scala/org/apache/spark/ui/viz/VisualizationListener.scala
@@ -25,7 +25,7 @@ import org.apache.spark.scheduler._
 import org.apache.spark.ui.SparkUI
 
 /**
- * A SparkListener that constructs RDD DAG visualization for the UI.
+ * A SparkListener that constructs the RDD DAG visualization for the UI.
  */
 private[ui] class VisualizationListener(conf: SparkConf) extends SparkListener {
   private val jobIdToStageIds = new mutable.HashMap[Int, Seq[Int]]
diff --git a/core/src/main/scala/org/apache/spark/ui/viz/VizGraph.scala b/core/src/main/scala/org/apache/spark/ui/viz/VizGraph.scala
index 6938a8d657cf7..2b3d8bc70be71 100644
--- a/core/src/main/scala/org/apache/spark/ui/viz/VizGraph.scala
+++ b/core/src/main/scala/org/apache/spark/ui/viz/VizGraph.scala
@@ -25,32 +25,40 @@ import org.apache.spark.rdd.RDDScope
 import org.apache.spark.scheduler.StageInfo
 
 /**
- * A representation of a generic scoped graph used for storing visualization information.
+ * A representation of a generic cluster graph used for storing visualization information.
  *
- * Each graph is defined with a set of edges and a root scope, which may contain children
- * nodes and children scopes. Additionally, a graph may also have edges that enter or exit
+ * Each graph is defined with a set of edges and a root cluster, which may contain children
+ * nodes and children clusters. Additionally, a graph may also have edges that enter or exit
  * the graph from nodes that belong to adjacent graphs.
  */
 private[ui] case class VizGraph(
     edges: Seq[VizEdge],
     outgoingEdges: Seq[VizEdge],
     incomingEdges: Seq[VizEdge],
-    rootScope: VizScope)
+    rootCluster: VizCluster)
 
+/** A node in a VizGraph. This represents an RDD. */
 private[ui] case class VizNode(id: Int, name: String)
+
+/** A directed edge connecting two nodes in a VizGraph. This represents an RDD dependency. */
 private[ui] case class VizEdge(fromId: Int, toId: Int)
 
-/** A cluster in the graph that represents a level in the scope hierarchy. */
-private[ui] class VizScope(val id: String, val name: String) {
+/**
+ * A cluster that groups nodes together in a VizGraph.
+ *
+ * This represents any grouping of RDDs, including operator scopes (e.g. textFile, flatMap),
+ * stages, jobs, or any higher level construct. A cluster may be nested inside of other clusters.
+ */
+private[ui] class VizCluster(val id: String, val name: String) {
   private val _childrenNodes = new ListBuffer[VizNode]
-  private val _childrenScopes = new ListBuffer[VizScope]
+  private val _childrenClusters = new ListBuffer[VizCluster]
 
   def this(id: String) { this(id, id.split(RDDScope.SCOPE_NAME_DELIMITER).head) }
 
   def childrenNodes: Seq[VizNode] = _childrenNodes.iterator.toSeq
-  def childrenScopes: Seq[VizScope] = _childrenScopes.iterator.toSeq
+  def childrenClusters: Seq[VizCluster] = _childrenClusters.iterator.toSeq
   def attachChildNode(childNode: VizNode): Unit = { _childrenNodes += childNode }
-  def attachChildScope(childScope: VizScope): Unit = { _childrenScopes += childScope }
+  def attachChildCluster(childCluster: VizCluster): Unit = { _childrenClusters += childCluster }
 }
 
 private[ui] object VizGraph extends Logging {
@@ -58,53 +66,56 @@ private[ui] object VizGraph extends Logging {
   /**
    * Construct a VizGraph for a given stage.
    *
-   * The root scope represents the stage, and all children scopes represent individual
-   * levels of RDD scopes. Each node represents an RDD, and each edge represents a dependency
-   * between two RDDs from the parent to the child.
+   * The root cluster represents the stage, and all children clusters represent RDD operations.
+   * Each node represents an RDD, and each edge represents a dependency between two RDDs pointing
+   * from the parent to the child.
+   *
+   * This does not currently merge common scopes across stages. This may be worth supporting in
+   * the future when we decide to group certain stages within the same job under a common scope
+   * (e.g. part of a SQL query).
    */
   def makeVizGraph(stage: StageInfo): VizGraph = {
     val edges = new ListBuffer[VizEdge]
     val nodes = new mutable.HashMap[Int, VizNode]
-    val scopes = new mutable.HashMap[String, VizScope] // scope ID -> viz scope
+    val clusters = new mutable.HashMap[String, VizCluster] // cluster ID -> VizCluster
 
-    // Root scope is the stage scope
-    val stageScopeId = s"stage_${stage.stageId}"
-    val stageScopeName = s"Stage ${stage.stageId}" +
+    // Root cluster is the stage cluster
+    val stageClusterId = s"stage_${stage.stageId}"
+    val stageClusterName = s"Stage ${stage.stageId}" +
       { if (stage.attemptId == 0) "" else s" (attempt ${stage.attemptId})" }
-    val rootScope = new VizScope(stageScopeId, stageScopeName)
+    val rootCluster = new VizCluster(stageClusterId, stageClusterName)
 
-    // Find nodes, edges, and children scopes that belong to this stage. Each node is an RDD
-    // that lives either directly in the root scope or in one of the children scopes. Each
-    // children scope represents a level of the RDD scope and must contain at least one RDD.
-    // Children scopes can be nested if one RDD operation calls another.
+    // Find nodes, edges, and operator scopes that belong to this stage
     stage.rddInfos.foreach { rdd =>
       edges ++= rdd.parentIds.map { parentId => VizEdge(parentId, rdd.id) }
       val node = nodes.getOrElseUpdate(rdd.id, VizNode(rdd.id, rdd.name))
 
       if (rdd.scope == null) {
-        // This RDD has no encompassing scope, so we put it directly in the root scope
+        // This RDD has no encompassing scope, so we put it directly in the root cluster
         // This should happen only if an RDD is instantiated outside of a public RDD API
-        rootScope.attachChildNode(node)
+        rootCluster.attachChildNode(node)
       } else {
-        // Otherwise, this RDD belongs to an inner scope
-        val rddScopes = rdd.scope
+        // Otherwise, this RDD belongs to an inner cluster,
+        // which may be nested inside of other clusters
+        val rddClusters = rdd.scope
           .split(RDDScope.SCOPE_NESTING_DELIMITER)
-          .map { scopeId => scopes.getOrElseUpdate(scopeId, new VizScope(scopeId)) }
-        // Build the scope hierarchy for this RDD
-        rddScopes.sliding(2).foreach { pc =>
+          .map { scopeId => clusters.getOrElseUpdate(scopeId, new VizCluster(scopeId)) }
+        // Build the cluster hierarchy for this RDD
+        rddClusters.sliding(2).foreach { pc =>
           if (pc.size == 2) {
-            val parentScope = pc(0)
-            val childScope = pc(1)
-            parentScope.attachChildScope(childScope)
+            val parentCluster = pc(0)
+            val childCluster = pc(1)
+            parentCluster.attachChildCluster(childCluster)
           }
         }
-        // Attach the outermost scope to the root scope, and the RDD to the innermost scope
-        rddScopes.headOption.foreach { scope => rootScope.attachChildScope(scope) }
-        rddScopes.lastOption.foreach { scope => scope.attachChildNode(node) }
+        // Attach the outermost cluster to the root cluster, and the RDD to the innermost cluster
+        rddClusters.headOption.foreach { cluster => rootCluster.attachChildCluster(cluster) }
+        rddClusters.lastOption.foreach { cluster => cluster.attachChildNode(node) }
       }
     }
 
     // Classify each edge as internal, outgoing or incoming
+    // This information is needed to reason about how stages relate to each other
     val internalEdges = new ListBuffer[VizEdge]
     val outgoingEdges = new ListBuffer[VizEdge]
     val incomingEdges = new ListBuffer[VizEdge]
@@ -120,21 +131,23 @@ private[ui] object VizGraph extends Logging {
       }
     }
 
-    VizGraph(internalEdges, outgoingEdges, incomingEdges, rootScope)
+    VizGraph(internalEdges, outgoingEdges, incomingEdges, rootCluster)
   }
 
   /**
    * Generate the content of a dot file that describes the specified graph.
    *
    * Note that this only uses a minimal subset of features available to the DOT specification.
-   * More style is added in the visualization later through post-processing in JavaScript.
+   * Part of the styling must be done here because the rendering library must take certain
+   * attributes into account when arranging the graph elements. More style is added in the
+   * visualization later through post-processing in JavaScript.
    *
-   * For the complete specification, see http://www.graphviz.org/Documentation/dotguide.pdf.
+   * For the complete DOT specification, see http://www.graphviz.org/Documentation/dotguide.pdf.
    */
   def makeDotFile(graph: VizGraph, forJob: Boolean): String = {
     val dotFile = new StringBuilder
     dotFile.append("digraph G {\n")
-    dotFile.append(makeDotSubgraph(graph.rootScope, forJob, indent = "  "))
+    dotFile.append(makeDotSubgraph(graph.rootCluster, forJob, indent = "  "))
     graph.edges.foreach { edge =>
       dotFile.append(s"""  ${edge.fromId}->${edge.toId} [lineInterpolate="basis"];\n""")
     }
@@ -144,25 +157,29 @@ private[ui] object VizGraph extends Logging {
     result
   }
 
-  /** Return the dot representation of a node. */
+  /**
+   * Return the dot representation of a node.
+   *
+   * On the job page, is displayed as a small circle without labels.
+   * On the stage page, it is displayed as a box with an embedded label.
+   */
   private def makeDotNode(node: VizNode, forJob: Boolean): String = {
     if (forJob) {
-      // On the job page, we display RDDs as dots without labels
       s"""${node.id} [label=" " shape="circle" padding="5" labelStyle="font-size: 0"]"""
     } else {
       s"""${node.id} [label="${node.name} (${node.id})"]"""
     }
   }
 
-  /** Return the dot representation of a subgraph recursively. */
-  private def makeDotSubgraph(scope: VizScope, forJob: Boolean, indent: String): String = {
+  /** Return the dot representation of a subgraph. */
+  private def makeDotSubgraph(scope: VizCluster, forJob: Boolean, indent: String): String = {
     val subgraph = new StringBuilder
     subgraph.append(indent + s"subgraph cluster${scope.id} {\n")
     subgraph.append(indent + s"""  label="${scope.name}";\n""")
     scope.childrenNodes.foreach { node =>
       subgraph.append(indent + s"  ${makeDotNode(node, forJob)};\n")
     }
-    scope.childrenScopes.foreach { cscope =>
+    scope.childrenClusters.foreach { cscope =>
       subgraph.append(makeDotSubgraph(cscope, forJob, indent + "  "))
     }
     subgraph.append(indent + "}\n")