There are a number of software and hardware technologies out there that enable snapshot backups ranging from Netapp and EMC disk array devices to snapshot capable file systems.

These technologies can represent a convenient way to do a backup and in some cases depending on the technology do a very quick restore of large databases. If these snapshots are performed on an active database, it is probably a good idea to put the data files in hot backup mode in order to freeze the data file headers. Before Oracle 10g, this had to be done on a tablespace level or data file level which could take a long time in some cases and introduce additional risk.

However, as of 10g you can put the ENTIRE database in hot backup mode using the command ALTER DATABASE BEGIN BACKUP;. No doubt this was introduced to make the use of oracle based snapshot backups easier (error checking, controlfile issues, etc.)

To take a database out of hot backup mode, use ALTER DATABASE END BACKUP;. This command was actually introduced in Oracle 9i but only worked when the database is mounted. The reason for this command was to facilitate a quick database start in case the database crashed while doing a snapshot backup or running a traditional backup.