SQL Server 2008 has a new feature for Auditing. You can audit anything happening on the SQL Server using this feature. Its a full pledged Audit implementation and it's very granular. You can audit the select queries of as single user on a single table in a database. It's that granular. Also its very simple to implement. You can audit all the DDL, DML and server login activities.
Change Data Capture is also a very good feature of SQL 2008. It's available only in Enterprise edition. It can be used to track the DDL happening on the tables.
Triggers can also be used. But that require considerable effort on your part to code them properly. If not written properly they cause lot of headaches.
I'd prefer CDC to Triggers. BTW, what are you planning to achieve ?