Normally you use the trigger functionality with Data Manipulation Language (DML) commands such as INSERT, UPDATE, and DELETE. Now SQL 2005 introduces new Data Definition Language (DDL) triggers. You can use it to audit schema changes.
Example of DDL Trigger:
CREATE TRIGGER safety
ON DATABASE
FOR DROP_TABLE, ALTER_TABLE
AS
PRINT ‘You must disable Trigger “safety” to drop or alter tables!’
ROLLBACK;
DISABLE TRIGGER safety ON DATABASE;
Please refer to MSDN for details: MSDN: Designing DDL Triggers.
An articel about using DDL triggers: Using SQL Server 2005 to document Sarbanes-Oxley compliance