VirtualBox - A Simple Guide to SnapShots...

Snapshot commands explained:
1) REVERT TO CURRENT SNAPSHOT (Ctrl+Shift+R)

This will discard changes in the "Current State" and go back to how the machine was in the LAST = MOST RECENT snapshot we've done, that is DAY 2.

2) DISCARD CURRENT SNAPSHOT AND STATE (Ctrl+Shift+B)

This will discard changes in the "Current State" AND in "LAST = MOST RECENT snapshot" we've done, that is DAY 2 and it will go back to how it was on DAY 1.

3) DISCARD SNAPSHOT (Ctrl + Shift + D)

This will be done without warning dialog! It will just delete the snapshot which is marked but keep the CURRENT STATE. The current state will have all changes up-to-date. If you want to delete all snapshots, you will have to do it to each of them. You can delete the snapshots in any order you like without effect on the current state.

More information including great examples: http://forums.virtualbox.org/viewtopic.php?f=1&t=23291