В BOL напмсано следующее, но разницы я не уловил

Autocommit transactions

This is the default mode for SQL Server. Each individual Transact-SQL statement is committed when it completes. You do not have to specify any statements to control transactions.

Implicit transactions

Set implicit transaction mode on through either an API function or the Transact-SQL SET IMPLICIT_TRANSACTIONS ON statement. The next statement automatically starts a new transaction. When that transaction is completed, the next Transact-SQL statement starts a new transaction