The snapshot represents the state of whatever was inside the VM at the time you took it - either just the virtual disks or also the memory (ie. the things you will be changing during your OS change - presumably an upgrade?)
yes I'm taking about OS upgrade, so incase of any issue after OS upgrade it is possible to restore snapshot taken before the activity.
That's a legitimate use of a snapshot - they're often used with OS patching for the same reason.
A few things worth noting:
- By default, every virtual disk of your VM will be included in the snapshot, and you cannot selectively only revert a single virtual disk back to a previous state - it's all or nothing - so be careful with other activity in your VM apart from your OS upgrade
- It would be good practice to delete the snapshot once you know you don't need to revert to it (ie. your upgrade went to plan)
- If you can, still do a backup first regardless of using a snapshot
yes, that's correct.
that snapshot will take all the information residing on the disk and memory of the VM.