MD5 Checksums

I have been seeing ever more so now when i download software the site gives me a MD5 checksum. Now im fairly sure this is to check file integrity but im not sure how to go about this. Is it like tripwire where it creates a checksum for the file then when you want to check it then you "scan" ( i cant think of a better word here) the file to get a checksum for it and compare this checksum with the one the website gave me? This is just a guess however also if this is so and I need software to use these checksums how do i go about getting it? and also just a little brain teaser, If i need software to check the integrity of stuff how do i check the integrity of the software used to check file integrity?:)

Yes, http://www.md5summer.org/ for windows or just md5 programtomakechecksumfor for most nix , more reading if interested http://www.google.com/search?hl=en&l...5+&btnG=Search

Thanks :)