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snapshot is a mechanism to preserve the state of a file at that particular time when snapshot is taken.
example:
time
t0: file: abcdef
t1: file: abczef
here d is changed to z. just store this difference in some where
t2: file: abczff
here e is changed to f. just store this difference in some where

now at time t > t2
the user have an ability to go back to any of the file state where snapshot is taken.

t1: present file abczff + changes e->f and d->z = abcdef
t2: present file abczff + changes e->f = abczef

Snapshot is point in time image of any individual file/directory or entire File System, this is normally done when we need to create point in time backup with almost 0 downtime of production server to take backup, during the snapshot all the disk I/O are cached and a block by block image is created of entire FS, before doing this a mapping/header of FS is created which gives the information about the which file lies in which block for the purpose of retrieval now this header and the image is copied to tape as backup.