md5sum is a command line program used to calculate MD5 hashes of files, and provides a simple way to check the integrity of a download.
Since any two given files are very unlikely to have the same MD5 hash sum unless they are exactly the same, the md5sum command line program is commonly used to verify download integrity, particularly for large files.
For example, it is generally recommended to run md5sum on a downloaded Ubuntu .iso file and comparing the result to the official hash for that particular image before creating a LiveCD/LiveUSB with it. This is to check for errors which could cause problems when trying to run or install it.
md5sum could also be used when the official hash for a piece of software is known and it is downloaded from somewhere other than the original source. This can help verify that the software is the particular version sought and that it is genuine (although when used for the latter purpose, it's worth noting that this is not a foolproof method; see link #2).
Links for further information:
- http://en.wikipedia.org/wiki/Md5sum
- http://en.wikipedia.org/wiki/Md5
- https://help.ubuntu.com/community/HowToMD5SUM
When using md5sum specifically to check Ubuntu .iso integrity, you can refer to a list of the correct MD5 hashes for each image version for all currently supported versions of Ubuntu here: https://help.ubuntu.com/community/UbuntuHashes
