A checksum is a number. It is a kind of redundancy check. There are different ways to calculate it. It serves as a check, that no errors have been made when writing down the number. In its simplest form, the digits are simply added up. This can however not detect errors of swapping digits around. Better ways (algorithms) of calculating a checksum are to use a Hamming code, a cyclic redundancy check or modular arithmetic. One of the uses of checksums is to check that account numbers have been entered correctly. 