Explanation
The SQL Server Audit feature enables you to audit server-level and database-level groups of events and individual events.
Audits can have the following categories of actions:
Server-level. These actions include server operations, such as management changes and logon and logoff operations.
Database-level. These actions encompass data manipulation languages (DML) and data definition language (DDL) operations.
Audit-level. These actions include actions in the auditing process.
References:
http://technet.microsoft.com/en-us/library/cc280663(v=sql.105).aspx