For CLR/CTS
http://msdn.microsoft.com/en-us/library/zw4w595w.aspx (CLR)
http://msdn.microsoft.com/en-us/library/2hf02550.aspx (CTS)
For OOPS concepts (does not include abstraction)
http://www.eggheadcafe.com/articles/pfc/oopbasics.asp
For ADO.NET concepts in brief
http://en.wikipedia.org/wiki/Ado.net
Abstraction concept: http://forums.msdn.microsoft.com/en-US/csharplanguage/thread/8ad621b8-a915-4d7e-89c3-5dbbc47202fd/
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Difference between Overloading and Overriding (http://www.dotnetspider.com/forum/ViewForum.aspx?ForumId=36)
OverLoading :- Method Name remains the same with different signatures.[mostly happens for operators]
Overriding : - Method name and signatures must be same.[mostly for functions]
Marshal Class
Provides a collection of methods for allocating unmanaged memory, copying unmanaged memory blocks, and converting managed to unmanaged types, as well as other miscellaneous methods used when interacting with unmanaged code.