With VirtualBox snapshots, you can save the current state of a virtual machine at any given time, to use it later. So, after taking a snapshot, you can at any time return to your system to a previous state, even if the virtual machine has been completely changed or damaged.
Snapshots are particularly useful to have a system clean and make sure that the virtual machine is free from bugs caused by the installation software, viruses or other.
Create a snapshot
If your virtual machine is already running: Select your virtual machine and "Machine"> "Take a snapshot"
If your virtual machine is stopped: Select your virtual machine of your choice (to the left) and then click the "snapshots" on the right, and then click on the "Take a snapshot" icon.
Enter a descriptive name (date and time will be automatically added), e.g "new installation" and then a description for your snapshot!
Click on Confirm.
The creation of the snapshot can take a little time.
Your snapshot appear in the list of snapshots. Below you will see "Current Status", which means that the current state of your virtual machine is derived from the previously created snapshot, it is based on it. If at that time you take another snapshots (# 2 and # 3), each of them will be a snapshot derived from the previous snapshot. You can create as many snapshots you want but it should be noted that snapshots use a lot of disk space!
How to use snapshots
Upon loading the snapshot, its the whole hard disk (e.g the ISO file) of the virtual machine which will be restored. Data may be overwritten in some cases!
As the name already suggest, the "Restore snapshot" option allow you to restore your system to a previous state.
This option allows you to delete a snapshot without changing the current state of the virtual machine.