Methods, Delegates, Predicates, Functions and Actions

The following is a simplistic cheat-sheet:

Methods: T X(T1 p1,... Tn pn)
A method is a named block of code that (optionally) accepts parameters and (optionally) returns values. (Honestly, if you don’t know what a method is, don’t bother read the rest of this – you have other stuff to learn first).
An anonymous method is a block of code that is not named, but instead is assigned directly to any of the delegate types mentioned below.

Delegate: delegate T X(T1 p1,... Tn pn)
A delegate is a reference to a method of type T with a particular signature (accepts parameters p1 through pn of types T1 through Tn). A member that is a delegate can be assigned, and reassigned, during run-time, with references to different methods of the same signature. A call to the member triggers a call to the referenced method.

Predicate: predicate X(T p)
A predicate is a specialized delegate that accepts a parameter p of type T and returns a Boolean.

Function: Func<T1,...Tn, R> X
Func is syntactic sugar for delegate R X<T1,...Tn>(T1 p1,... Tn pn) – a delegate for a method that returns a value of type R and accepts parameters of type T1 through Tn.

Action: Action<T1,...Tn> X
Action is syntactic sugar for delegate void X<T1,...Tn>(T1 p1,... Tn pn) – a delegate for a method that does not return a value but accepts parameters of type T1 through Tn.

Do you want to assign different algorithms to the same procedural call?
-> Yes: Does the algorithm need to return a value?

–> Yes: Will the algorithm test an input parameter and return a bool?

—> Yes: consider using Predicate
—> Otherwise: use a Func.

–> No: Use an Action

-> No: use a method.

You can see more about when to use each here:
http://stackoverflow.com/questions/4317479/func-vs-action-vs-predicate
http://stackoverflow.com/questions/2282476/actiont-vs-delegate-event