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 a Hamming code, or a cyclic redundancy check. One of the uses of checksums is account numbers. The Swiss postal account, for example, has checksum built in. Another place where checksums are used are serial numbers for computer software.