About Time Machine's 'local snapshots' on Mac notebooks
Time Machine in OS X Mountain Lion and Lion includes a feature called "local snapshots" that keeps copies of files you create, modify or delete on your internal disk.
Local snapshots compliment regular Time Machine backups (that are stored on your external disk or Time Capsule), giving you a "safety net" for times when you might be away from your external backup disk or Time Capsule and accidentally delete a file.
On Mac notebook computers, local snapshots are automatically enabled when you turn on Time Machine. Just like regular Time Machine backups, local snapshots are made hourly in the background.
Note: The Time Machine status in the menu bar does not change when a local snapshot is created.
Local snapshots are periodically condensed into daily or weekly snapshots to minimize the space used on your disk. If your disk is low on space, Time Machine stops creating new snapshots, and some or all existing snapshots may be removed to make space available for applications to use. If sufficient disk space becomes available again, Time Machine resumes creating local snapshots. This means your disk will have the same amount of available space as it would if Time Machine were not enabled.
When you enter the Time Machine browser (used to restore data), local snapshots will appear on the timeline along with regular backups, distinguished by different colors. Gray tick marks represent local snapshots and pink tick marks represent backups stored on your external backup disk or Time Capsule. Note: Pink tick marks will be dimmed if your notebook is not connected to your external backup disk or Time Capsule.
In the "About This Mac" window, the Storage tab lets you discover how much space your local snapshots are using for backups.
Note: You may notice a difference in available space statistics between Disk Utility, Finder, and Get Info inspectors. Those differences are expected and can be safely ignored. The Finder displays the available space on the disk without accounting for the local snapshots, because local snapshots will surrender their disk space if needed.