Time Machine includes a feature called "local snapshots" in OS X Lion and later. This feature keeps copies of files you create, modify or delete on your internal disk when your backup drive is not available.
What are local snapshots?
Local snapshots complement regular Time Machine backups that are stored on your external disk or Time Capsule by creating a local backup on your startup disk when your normal backup drive is not available. This provides you with a "safety net" for times when you might be away from your external backup disk or Time Capsule but accidentally delete a file. When your normal backup is available again, Time Machine copies the local snapshot contents from your startup disk to your normal backup drive.
On Mac notebook computers, local snapshots are automatically enabled when you turn on Time Machine.A single daily snapshot is saved for every 24 hours, counting from the time you start or restart your computer. Similarly, a single weekly snapshot is saved for one week. If you want Time Machine to stop saving local snapshots, open Time Machine preferences and slide the switch to Off. Snapshots resume when you turn Time Machine back on.
Note: The Time Machine status in the menu bar does not change when a local snapshot is created or modified.
When you enter the Time Machine browser (used to restore data), local snapshots 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 appear dimmed if your notebook is not connected to your external backup disk or Time Capsule.
Do local snapshots take up disk space?
Time Machine is designed to store local snapshots only when there is plenty of free disk space on the startup disk. This means that you can keep using your available disk space when you need it.
If your disk is low on space, Time Machine stops creating new snapshots. Some or all 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 has the same amount of available usable space as it would if Time Machine were not enabled. Time Machine uses the rules below to determine whether to stop creating snapshots, or to remove existing snapshots.
Checking free disk space
You can see how much space is being used by local snapshots from the "About This Mac" window.
Select About This Mac from the Apple menu.
Click the More Info button in the window that appears.
Click the Storage tab to see available and used disk space.
The space used by local snapshots is labeled as 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 local snapshots, because local snapshots surrender their disk space as needed