A transaction is the unit of work exchanged between the client and server.
The log records for a given transaction are moved into stable storage when the transaction is committed. The database information that is stored on disk remains consistent because the server ensures that the recovery log records, which represent the updates to these database pages, are written to disk.
During restart-recovery, the server uses the active and archive log information to maintain the consistency of the server by redoing and, if necessary, undoing ongoing transactions from the time that the server was halted. The transaction is then committed to the database.
Transaction commit is a function of all the log records for that transaction being written to the recovery log. This function ensures that the necessary redo and undo information is available to replay these transaction changes against the database information.