A checksum is a small piece of data derived from a larger set of data, used to verify the integrity of that data. It is created by applying a mathematical algorithm to the original data, producing a unique value. When the data is transmitted or stored, the checksum can be recalculated and compared to the original to check for errors or corruption. Checksums are commonly used in computer networking and data storage. For example, when downloading files, a checksum can ensure that the file has not been altered or damaged during the transfer. If the checksums match, the data is likely intact; if not, it may need to be resent or repaired.