Hi Aditya
Trigger.NewMap: Trigger.newMap returns map of new records which are trying to insert into Database. This is available in Before Insert, Before Update, After Insert, After Update Triggers and undelete Triggers. This list of records can only modified in Before triggers.
Trigger.OldMap: Trigger.oldMap returns map of old records which are updated with new values. These List of records already there in Database. Trigger.oldmap available in Before update, after update, Before Delete and After Delete triggers.