I think he's talking about differential backups...because a snapshot would be like a full backup...
A snapshot is a term used to restore a system to a given state, just as if you took a photo of it and said, ok, now be like this.
The easiest way to do this is just make a full copy of everything you have and store it on the side.
A better way is to use a system that takes a "base measurement" and then just records the changes. So what I have, as an example, is a directory called "backup" and in it are individual directories for each day. I can go into that day's directory, and the whole file system inside it looks exactly like it did on that day. What is really happening is that the unchanged files are just hardlinks (Windows kernel actually supports this too, why can't they trust their users with such cool tools?) so its essentially just a filename in the later directories pointing back to the original file. When a file changes, it doesn't get linked but its new version is copied.
Since my files don't change much, I have 10gb that I back up, with daily changes from 30 days, and it takes up about 11gb worth of space only.
I'm pretty sure that Vista has incorporated similar functionality from Server2003 which is even better in that it records only file deltas (the actual change) on each file save.
So in answer to above comment, yes differential backups done in a way to provide easy access to a system snapshot. BTW, you can see the same principle used by VMware's workstation product. The "snapshots" build off of the base OS image and as such are useless without it.