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A tape drive uses a tape, similar to cassettes used for music. A tape is serial in nature, which means that in order to get to any given part of it, you first have to go through all the parts in between. A disk can be accessed randomly, i.e., you can jump directly to any place on the disk. The serial access of tapes makes them slow.
On the other hand, tapes are relatively cheap to make, since they do not need to be fast. They can also easily be made quite long, and can therefore contain a large amount of data. This makes tapes very suitable for things like archiving and backups, which do not require large speeds, but benefit from low costs and large storage capacities.