diff --git a/dvc/api.py b/dvc/api.py
index 48a24c494b..def9e8cf5b 100644
--- a/dvc/api.py
+++ b/dvc/api.py
@@ -7,7 +7,7 @@
 
 
 class UrlNotDvcRepoError(DvcException):
-    """Thrown if given url is not a DVC repository.
+    """Thrown if given URL is not a DVC repository.
 
     Args:
         url (str): URL to the repository
diff --git a/dvc/command/data_sync.py b/dvc/command/data_sync.py
index 9442e2bbbe..c12b1afcac 100644
--- a/dvc/command/data_sync.py
+++ b/dvc/command/data_sync.py
@@ -148,7 +148,8 @@ def add_parser(subparsers, _parent_parser):
     pull_parser.set_defaults(func=CmdDataPull)
 
     # Push
-    PUSH_HELP = "Push data files to a DVC remote storage."
+    PUSH_HELP = "Uploads files or directories tracked by DVC"
+    " to remote storage."
 
     push_parser = subparsers.add_parser(
         "push",
@@ -191,7 +192,8 @@ def add_parser(subparsers, _parent_parser):
     push_parser.set_defaults(func=CmdDataPush)
 
     # Fetch
-    FETCH_HELP = "Fetch data files from a DVC remote storage."
+    FETCH_HELP = "Get files or directories tracked by DVC from remote storage"
+    " into the cache."
 
     fetch_parser = subparsers.add_parser(
         "fetch",
diff --git a/dvc/command/diff.py b/dvc/command/diff.py
index 6505555725..5667adfe56 100644
--- a/dvc/command/diff.py
+++ b/dvc/command/diff.py
@@ -120,8 +120,9 @@ def run(self):
 
 def add_parser(subparsers, parent_parser):
     DIFF_DESCRIPTION = (
-        "Compare two versions of the DVC repository.\n"
-        "Shows the list of paths added, modified, or deleted"
+        "Show changes between commits in the DVC repository,"
+        " or between a commit and the workspace."
+        "Prints the list of paths added, modified, or deleted."
     )
     diff_parser = subparsers.add_parser(
         "diff",
diff --git a/dvc/command/metrics.py b/dvc/command/metrics.py
index de11e3e9ca..6cb3887fd7 100644
--- a/dvc/command/metrics.py
+++ b/dvc/command/metrics.py
@@ -273,8 +273,8 @@ def add_parser(subparsers, parent_parser):
     metrics_remove_parser.add_argument("path", help="Path to a metric file.")
     metrics_remove_parser.set_defaults(func=CmdMetricsRemove)
 
-    METRICS_DIFF_HELP = "Show a table of changes between metrics among "
-    "versions of the DVC repository."
+    METRICS_DIFF_HELP = "Show changes in metrics between commits"
+    " in the DVC repository, or between a commit and the workspace."
     metrics_diff_parser = metrics_subparsers.add_parser(
         "diff",
         parents=[parent_parser],
diff --git a/scripts/completion/dvc.zsh b/scripts/completion/dvc.zsh
index 4549306ac9..22084cb765 100644
--- a/scripts/completion/dvc.zsh
+++ b/scripts/completion/dvc.zsh
@@ -12,14 +12,14 @@
 
 _dvc_commands() {
   local _commands=(
-    "add:Take data files or directories under DVC control."
+    "add:Track data files or directories with DVC."
     "cache:Manage cache settings."
     "checkout:Checkout data files from cache."
     "commit:Save changed data to cache and update DVC-files."
     "config:Get or set config settings."
     "destroy:Remove DVC-files, local DVC config and data cache."
-    "diff:Show a diff of a DVC controlled data file or a directory."
-    "fetch:Fetch data files from a DVC remote storage."
+    "diff:Show changes between commits in the DVC repository, or between a commit and the workspace."
+    "fetch:Get files or directories tracked by DVC from remote storage into the cache."
     "get-url:Download or copy files from URL."
     "get:Download data from DVC repository."
     "gc:Garbage collect unused objects from cache or remote storage."
@@ -100,8 +100,8 @@ _dvc_destroy=(
 _dvc_diff=(
   "--show-json[Format the output into a JSON]"
   "--checksums[Display checksums for each entry]"
-  "1:Git reference to the older version:"
-  "2:Git reference to the newer version:"
+  "1:Old Git commit to compare (defaults to HEAD):"
+  "2:New Git commit to compare (defaults to the current workspace):"
 )
 
 _dvc_fetch=(