MD5 (MD5 message-digest algorithm) is a hash function that can be used to verify the integrity of a file, by comparing the hash of the original file to the hash of the transferred file. If these values are different, then the two files are not exactly the same. You therefore have to download again your file.

You can use WinMD5Free utility available on the following webpage

Open a Terminal (located in /Applications/Utilities/) and type md5 command followed by the path to your file name (you can drag and drop your file from the Finder into the Terminal window to display the path to your file): md5 path/to/file

Most Linux distributions come with the md5sum utility, thus the following command can be used: md5sum path/to/file