operating system A copy of all files which have changed

since the date of some previous backup. The first backup of a

file system should include all files - a "full backup".

Call this level 0. The next backup could also be a full level

0 backup but it is usually much quicker to do a level 1 backup

which will include only those files which have changed since

the level 0 backup. Together the level 0 and level 1 backups

will include the latest version of every file. Level 1

backups can be made until, say, the backup tape is nearly

full, after which we can switch to level 2. Each level

includes those files which have changed since the last backup

at a lower level. The more levels you use, the longer it will

take to restore the latest version of a file (or all files) if

you don't know when it was last modified.