What is an MD5 Sum?
An MD5 sum can be used to generate a digital fingerprint of a file. This can be used to make sure a file you have downloaded is exactly the same as the one online. This prevents fraud and highlights any download errors. To check the file, simply run the md5sum utility on it, e.g.:
md5sum ubuntusatanic-666.4-i386.iso 5a322ca977b06f1c2bb81a1b7dbd13ad ubuntusatanic-666.4-i386.iso
...and compare the output against the original md5sum - usually supplied with the file or available at the download site. Here's the one for the SE 666.4 ISOs: