Some file systems can create snapshots of their states. The state of a file system changes constantly. Every time a file is updated, copied or removed, the state of the file system changes. A non-snapshotting file system can only keep track of the most recent (current) state.
In other words, if you change a file, the old version is gone.
A snapshotting file system, however, has the ability to maintain a history of states of the file system over time. When a file is updated, its old version is maintained. In fact, a snapshotting file system may track all versions of a file since it is created.
Because a directory is really a special file, this means that when a file is “deleted”, the containing directory is modified. A snapshotting file system can also maintain versions of a directory, and therefore keep track of “deleted” files.
One can view snapshotting as a method that continuously back up a file system with respect to keeping track of changes.
Note that snapshotting is a requirement for some systems used by the government.