A client connecting to a server is an example of an event, the defined function within a plugin onClientConnect is an example of a registered forward.
http://wiki.alliedmods.net/Game_Events_(Source)
http://wiki.alliedmods.net/Hidden:_Source_Events http://wiki.alliedmods.net/Generic_Source_Server_Events http://wiki.alliedmods.net/Generic_Source_Server_Events http://wiki.alliedmods.net/Counter-Strike:_Source_Events