Mac OS X Lion introduces a new TimeMachine backup feature for the MacBook Air and Pro notebooks, called MobileBackups. It is a local version of TimeMachine backups that enables local snapshots of your data while you’re mobile and away from your main TimeMachine backups. On the TimeMachine timeline screen, local backups are shown as white ticks, while main backups are purple.
This is a neat feature, however it eats up your hard disk space and can grow to tens of gigabytes very fast. Thankfully, you can disable it permanently or temporarily by using the tmutil program.