check online, there are tons of md5 sum tutorials. basically it's a way of creating a string of number/characters that checks the integrity of any downloaded files..
there are tons of free md5 sum checker as well
__________________
lisp hacker

running: FreeBSD 5.4 - still learning

develop with: SBCL + emacs for lisp, Anjuta IDE +gcc for c, SPE for python..
browse with: opera