Sequence of Trigger and automation rules run in Salesforce.com

It's important to know that what thing will be executed first when any record is about insert or delete or update. So let's have a look on all execution in their execution order.

1. Old record loaded from database (or initialized for new inserts)
2. New record values overwrite old values
3. System Validation Rules
4. Before triggers 
5. Custom Validation Rules
6. Record saved to database 
7. Record reloaded from database
8. After triggers 
9. Assignment rules
10. Auto-response rules
11. Workflow rules
12. Escalation rules
13. Parent Rollup Summary Formula value updated (if present)
14. Database commit
15. Post-commit logic (sending email)