As changes are made to the contents of a database, a rollback log is kept for the purpose of canceling changes if a transaction is rolled back or if a transaction is uncommitted when a system failure occurs. There is a separate rollback log for each connection. When a transaction is committed or rolled back, the rollback log contents for that connection are deleted. The rollback logs are stored in the database, and rollback log pages are copied into the checkpoint log along with other pages that are changed.
The rollback log is also called the undo log.
For more information about transaction processing, see Using Transactions and Isolation Levels.