Apex Execution Order

1. Loads the original record from the database or initializes the record for an upsert statement.
2. Loads the new record field values from the request and overwrites the old values. If the request came from a standard UI edit page, Salesforce runs system validation to check the record for:
* Compliance with layout-specific rules
* Required values at the layout level and field-definition level
* Valid field formats
* Maximum field length
Salesforce does not perform system validation in this step when the request comes from other sources, such as an Apex application or a Web Services API call.
3. Executes all before triggers.
4. Runs most system validation steps again, such as verifying that all required fields have a non-null value, and runs any user-defined validation rules. The only system validation that Salesforce does not run a second time (when the request comes from a standard UI edit page) is the enforcement of layout-specific rules.
5. Saves the record to the database, but does not commit yet.
6. Executes all after triggers.
7. Executes assignment rules.
8. Executes auto-response rules.
9. Executes workflow rules.
10. If there are workflow field updates, updates the record again.
11. If the record was updated with workflow field updates, fires before and after triggers one more time (and only one more time).
12. Executes escalation rules.
13. If the record contains a roll-up summary field or is part of a cross-object workflow, perform calculations and updates the roll-up summary field in the parent record.
14. Commits all DML operations to the database.
15. Executes post-commit logic, such as sending an email.

Triggers and Order of Execution

When a record is saved with an insert, update, or upsert statement, the following events occur in order:

1. The original record is loaded from the database (or initialized for an insert statement)
2. The new record field values are loaded from the request and overwrite the old values
3. All before triggers execute
4. System validation occurs, such as verifying that all required fields have a non-null value and running any user-defined validation rules
5. The record is saved to the database, but not yet committed
6. All after triggers execute
7. Assignment rules execute
8. Auto-response rules execute
9. Workflow rules execute
10. If there are workflow field updates, the record is updated again
11. If the record was updated with workflow field updates, before and after triggers fire one more time (and only one more time)
12. Escalation rules execute
13. All DML operations are committed to the database
14. Post-commit logic executes, such as sending email

Salesforce Apex Manual

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