If I wan to do some activity after sign on.how can you do that?

Suppose I have requirement that Immediately  after user sign in we need get some data from
an ERP system. how can we do that?

We can use Liferay's Hooks.  Hooks can be used to perform custom actions on certain common portal events such as user log in or system start up.

we can define actions for events in portal.properties.
 #
 # Login event
 #

 login.events.pre=com.suresh.MyLoginAction
login.events.post

  #
  # Logout event
  #

 logout.events.pre=  

logout.events.post=