Trigger Before and After Explanation

• Before triggers can be used to update or validate record values before they are saved to the database.
• After triggers can be used to access field values that are set by the database (such as a record's Id or lastUpdated field), and to affect changes in other records, such as logging into an audit table or firing asynchronous events with a queue.

http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_triggers.htm