There are two basic commands for snapshots; one makes a snapshot with a given name, the other retrieves a named snapshot.
C-x v s name RET
Define the last saved versions of every registered file in or under the
current directory as a snapshot named name
C-x v r name RET
Check out all registered files at or below the current directory level
using whatever versions correspond to the snapshot name
This command reports an error if any files are locked at or below the current directory, without changing anything; this is to avoid overwriting work in progress.
A snapshot uses a very small amount of resources—just enough to record the list of file names and which version belongs to the snapshot. Thus, you need not hesitate to create snapshots whenever they are useful.
You can give a snapshot name as an argument to C-x v = or C-x v ~ (see Old Versions). Thus, you can use it to compare a snapshot against the current files, or two snapshots against each other, or a snapshot against a named version.