Base keyword and This keyword

Firstly, they can be used for 'constructor chaining' where one instance constructor calls another.

'this' refers to another constructor in the current class and 'base' refers to a constructor in the base class.

Secondly, 'this' can be used within any instance member of the class to refer to the current instance.

'base' can likewise be used within any instance member of the class to call a member of the base class that would be otherwise be hidden or overridden by a similarly named member of the current class. 

Thirdly, 'this' can be used to define an indexer of the current class.

For more details see:

http://msdn.microsoft.com/en-us/library/dk1507sz(VS.71).aspx  

http://msdn.microsoft.com/en-us/library/hfw7t1ce(VS.71).aspx