http://hegel.ittc.ukans.edu/topics/linux/man-pages/man1/md5sum.1.html Its an error correction utility...every file that your do a md5sum creates a fingerprint hash or a code that if you compare a file to the sum and its the same you can be assured the file you have is correct and not corrupted/virused/bogus
Its the same as a CRC check (cyclic redundancy check) $20 word but all it means is it checks to ensure what is sent is what is actually recieved
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Cyclic redundancy checking is a method of checking for errors in data that has been transmitted on a communications link