How do protocols in Objective C work? [duplicate]

This question already has answers here: Clos开发者_运维问答ed 11 years ago.

Possible Duplicate:

Objective-c - Purpose of categories and protocols

Can anyone justify @protocol mechanism of Objective C in simple words. Like,

1. Defining protocol
2. Implementing protocol
3. Other alternatives of protocol

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You can find a good introduction on protocols at Apple's developer site, as well as on WikiBooks.