What do the contents of PowerShell's $PSVersionTable represent?

The PowerShell built-in help describes $PSVersionTable:

Get-Help about_Automatic_Variables

You may need to run Update-Help first if you have not run it before.

What's great about Get-Help is you don't have to know exactly what you're looking for. What I typed to find this topic first was actually just help PSVersionTable, and it listed all help topics that contained "PSVersionTable" (there are a few others).

This is the section of about_Automatic_Variables that describes $PSVersionTable, as currently returned by PowerShell for me, but it's the least important part, as the OP noted:

$PsVersionTable Contains a read-only hash table that displays details about the version of Windows PowerShell that is running in the current session. The table includes the following items:

CLRVersion:                The version of the common language runtime (CLR)

BuildVersion:              The build number of the current version

PSVersion:                 The Windows PowerShell version number

WSManStackVersion:         The version number of the WS-Management stack

PSCompatibleVersions:      Versions of Windows PowerShell that are 
                           compatible with the current version

SerializationVersion:      The version of the serialization method

PSRemotingProtocolVersion: The version of the Windows PowerShell remote 
                           management protocol