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System.Management.Automation.Cmdlet (abstract class)

System.Management.Automation.Cmdlet is the base class for all PowerShell cmdLets.
Cmdlet derives from System.Management.Automation.Internal.InternalCommand.

Comparison Cmdlet to PSCmdlet

The Cmdlet class provides some common functionality for cmdLets, such as object output and record processing.
Thus, deriving a class from Cmdlet is the common way to create a user defined cmdLet.
A derived class is System.Management.Automation.PSCmdlet. If a cmdLet-class is derived from PSCmdlet, the cmdLet also has access to the MSH runtime, including variables in the session state, access to the host or information about the current cmdlet providers.

BeginProcessing(), ProcessRecord() and EndProcessing() methods

When deriving a class from Cmdlet to create a new command, the three methods BeginProcessing(), ProcessRecord() and EndProcessing() can be overridden to initialize command execution, process each object that the command receives from the pipeline, and clean up command execution.
These three methods correspond to the begin, process and end blocks in PowerShell functions.

Write* methods

The Cmdlet has a few Write* methods:
For example, the WriteProgress method accepts instances of System.Managment.Automation.ProgressRecord.
WriteError() sets $? to $false.

ThrowTerminatingError

Calling ThrowTerminatingError() raises a terminating error.
Compare with WriteError() which allows the cmdlet to continue.
ThrowTerminatingError() sets $? to $false.

Simple C-Sharp example

The following simple source code demonstrates how a cmdLet can be written in C#: 
//
//    csc -r:"C:\Program Files (x86)\Reference Assemblies\Microsoft\WindowsPowerShell\3.0\system.management.automation.dll" -target:library Add-Strings.cs
//
//    import-module .\Add-Strings.dll
//
//    Add-Strings foo bar
//
using System.Management.Automation;

[Cmdlet("Add", "Strings")]
[OutputType("System.String")]
public class AddStrings : Cmdlet {

 //
 // Specify the two parameters for the cmdlet:
 //
   [Parameter(Position=0, Mandatory=true)]
    public string Text1 {
       get { return t1;  }
       set { t1 = value; }
    }

   [Parameter(Position=1, Mandatory=true)]
    public string Text2 {
       get { return t2;  }
       set { t2 = value; }
    }

    private string t1;
    private string t2;

 //
 // Define functionality of cmdlet by
 // overriding ProcessRecord.
 // This method will be called when the
 // cmdlet is executed.
 //
    protected override void ProcessRecord() {
 //
 //    WriteObject(…) puts an object into
 //    the output pipe:
 //
       WriteObject(Text1 + " " + Text2);
    }
}
Github repository .NET-API, path: /System/Management/Automation/Cmdlet/Add-Strings.cs
The source code can be compiled with into a DLL (Add-Strings.dll) with: 
csc.exe -r:"C:\Program Files (x86)\Reference Assemblies\Microsoft\WindowsPowerShell\3.0\system.management.automation.dll" -target:library Add-Strings.cs
This DLL must then be imported into a session with 
import-module .\Add-Strings.cs
With this import, the cmdlet is available to be used: 
Add-Strings foo bar

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