Apple's Time Machine structure is described by "realms", which parallel the directory hierarchy of the backup files. There are several realms, which are listed and described below:

backup source

A volume currently being backed up by Time Machine.

backup disk

The HFS+ volume that contains Time Machine backups.

backup destination

In the case of a local destination, a synonym for backup disk. For network destinations, this is the AFP or SMB share on which the backup disk image resides.

backup disk image (or backup image)

A sparse bundle that, when mounted, is the backing store for a volume that is a backup disk.

backup store

The top-level "Backups.backupdb" directory at the root of a backup disk.

/Volumes/Chronoton/Backups.backupdb

machine directory

A directory inside a backup store that contains all the backups for a particular computer. For local destinations, a backup store can contain multiple machine directories, all for separate computers.

/Volumes/Chronoton/Backups.backupdb/thermopylae

snapshot

A directory inside a machine directory that represents a single initial or incremental backup of one computer. The word "snapshot", in most contexts, is a generic term and is not to be confused with a "local Time Machine snapshot", which is simply a snapshot stored locally on the computer.

/Volumes/Chronoton/Backups.backupdb/thermopylae/2011-07-03-123456

snapshot volume

A directory inside a snapshot that represents a single initial or incremental backup of one backup source.

/Volumes/Chronoton/Backups.backupdb/thermopylae/2011-07-03-123456/Mac HD