I came across this page in the Apex documentation and wanted to share it with everyone. So many people have asked about this in the past, so it seems a good idea to publicize it:
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
Please note the following when working with triggers:
* When Enable Validation and Triggers from Lead Convert is selected, if the lead conversion creates an opportunity and the opportunity has Apex before triggers associated with it, the triggers run immediately after the opportunity is created, before the opportunity contact role is created. For more information, see “Customizing Lead Settings” in the Salesforce online help.
* If you are using before triggers to set Stage and Forecast Category for an opportunity record, the behavior is as follows:
o If you set Stage and Forecast Category, the opportunity record contains those exact values.
o If you set Stage but not Forecast Category, the Forecast Category value on the opportunity record defaults to the one associated with trigger Stage.
o If you reset Stage to a value specified in an API call or incoming from the user interface, the Forecast Category value should also come from the API call or user interface. If no value for Forecast Category is specified and the incoming Stage is different than the trigger Stage, the Forecast Category defaults to the one associated with trigger Stage. If the trigger Stage and incoming Stage are the same, the Forecast Category is not defaulted.