A checksum is a value used to verify the integrity of a file or a data transfer. In other words, it is a sum that checks the validity of data. Checksums are typically used to compare two sets of data to make sure they are the same. Some common applications include verifying a disk image or checking the integrity of a downloaded file. If the checksums don't match those of the original files, the data may have been altered or corrupted.
A checksum can be computed in many different ways, using different algorithms. For example, a basic checksum may simply be the number of bytes in a file. However, this type of checksum is not very reliable since two or more bytes could be switched around, causing the data to be different, though the checksum would be the same. Therefore, more advanced checksum algorithms are typically used to verify data. These include cyclic redundancy check (CRC) algorithms and cryptographic hash functions.
It is rare that you will need to use a checksum to verify data, since many programs perform this type of data verification automatically. However, some file archives or disk images may include a checksum that you can use to check the data's integrity. While it is not always necessary to verify data, it can be a useful means for checking large amounts of data at once. For example, after burning a disc, it is much easier to verify that the checksums of the original data and the disc match, rather than checking every folder and file on the disc.
Both Mac and Windows include free programs that can be used to generate and verify checksums. Mac users can use the built-in Apple Disk Utility and Windows users can use the File Checksum Integrity Verifier (FCIV).