Behaviours are like patterns which makes easier to understand the erlang code to every Erlang programmer. The idea behind the Behaviour is to divide the Erlang code for a process in a generic part (means: a behaviour module) and a callback module.

1. gen_server: gen_server allows to send and receive sync and async messages, start and stop processes and allows to code change.

2. gen_event: gen_event is used for implementing event handling functionality and the OTP event handling model consists of a generic event manager process with an arbitrary number of event handlers that are added and deleted dynamically.

3. supervisor: supervisor is used for monitoring processes and we can implement a good shutdown and restart strategy around it.

4. gen_fsm: gen_fsm allows changes whether there are too many states involve in an application.

5. gen_mod: gen_mod provides the facility to extend the ejabberd modules with the xmpp server have to implement gen_mod behaviour. And also it has start/2 and stop/1 functions that allows us to intergrate the custom functions of our module.