What is difference between Delegates and function?

There are at least 2 scenarios where delegates are useful in 'real world' programming:

1. Because delegates are 'multi-cast' you can call several methods at once with just one invocation of the delegate. 

This is why delegates are used as the basis of events because several 'event handlers' can be added to the event's invocation list and called one after the other when the event fires. 

2. Delegates enable you to pass methods as arguments to another method. Any method which satisfies the delegate signature can be passed in this way including lambda expressions.

One use for this is callbacks i.e. where a method needs to 'call back' another method with some results, say.

Another is to modify the operation of the method to which it is passed. This is used a lot in the LINQ exension methods.

For example, if a method is passed a collection of ints and performs some operation on them, a delegate could also be passed which constrained the operation of the method to only those ints which were even numbers, were less than 100 or filtered them in some other way.