A checksum is a way to check if chunk of information transmitted over a network has been transmitted without any data corruption. A checksum is similar to CRC (cyclic redundancy check) or parity checks to make sure that data transmitted over network is received without corruption. Checksum involves generating a key based on the content of the transmission. The receiving party can apply the same algorithm again and compare with the key. If both of them are equal then the transmission is corruption free. Checksums are used when large files are downloaded. Md5 Checksum is the most widely used one based on MD5 algorithm. For example take a look at the MySQL downloads page, you can see the MD5Checksum under every downloadable link. There are free tools that can generate the checksum based on the downloaded file to compare with the key. WinMD5Sum is one tool that you can download for windows and md5sum is a command tool that comes with UNIX Linux systems that can be used directly.