add documentation on problems #12002

framework/doc/content/syntax/Problem/index.md

@@ -1,4 +1,35 @@
-<!-- MOOSE Documentation Stub: Remove this when content is added. -->
+# Problem system overview
+
+The Problem class is one of the core extension points in MOOSE. Problems
+are designed to hold the `EquationSystems` objects (from libMesh) that
+house the numerical systems we are ultimately solving for our computing.
+
+Fortunately when you are first getting started with MOOSE you normally
+don't have to worry or do anything special with the Problem object. MOOSE
+automatically constructs a suitable Problem for your simulation type
+taking into account the other types of objects present in the input file.
+Most simulations use the `FEProblem` class, which contains a single
+`NonlinearSystem` and single `AuxiliarySystem` or a single `MooseEigenSystem`
+and single `AuxiliarySystem`. The `NonlinearSystem` or `MooseEigenSystem`
+contains the matrix and vectors used for solving the equations implemented
+through a combination of other objects in your simulations (`Kernels`, `BCs`,
+etc.). The `AuxiliarySystem` houses the solution vectors use to hold
+computed solutions or values.
+
+As your application grows in complexity, you may find it useful or
+necessary to create your own problems to extend default behavior provided
+by the core MOOSE framework. Common examples include, specialized
+convergence tests, etc.
+
+# Automatic Problem Creation
+
+The automatic problem creation is handled for you by MOOSE. In a normal
+input file that does not contain a special `[Problem]` block, MOOSE
+will create a suitable Problem for you. If however, you need to change
+specific system related parameters you may find yourself adding a
+`[Problem]` block with name/value pairs. Different types of Problems
+may be instantiated by using the `/Problem/type` whose default value is
+`FEProblem`.
 
 # Problem System
 
@@ -7,4 +38,3 @@
 !syntax list /Problem objects=False actions=False subsystems=True
 
 !syntax list /Problem objects=False actions=True subsystems=False
-