Trigger.new : Returns a list of the new versions of the sObject records. Note that this sObject list is only available in insert and update triggers, and the records can only be modified in before triggers.
Trigger.old : Returns a list of the old versions of the sObject records. Note that this sObject list is only available in update and delete triggers.
For more info visit the below link
http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_triggers_context_variables.htm
Cheers!!!
