Difference between NSOperation and NSInvocationOperation?

Question

1) what is happening when we add NSOperation or NSInvocationOperation in NSoperationQueue?

2) What is the difference between

Answer 1

Cocoa Is My Girlfriend has a good tutorial on the use of NSOperation and NSOperationQueue. The tutorial makes use of NSOperation to download several webpages simultaneously in separate threads.

Also Check this question NSOperation on the iPhone