md5sums are character strings that are the result of running the md5sum program against a particular file. An md5sum can often be found on download sites such as linuxiso.org where you can get installation cds for Linux. After downloading the file, you run md5sum on it to see if the result is the same as the result from md5sum on the file on the site. If the strings match, the files are the same. If what you get does not match what is listed as the md5sum on the site, then you have a corrupt file.

Some cd-recording applications (e.g. k3b) calculate the md5sum for you before burning iso-images, so you do not need to do so yourself.

md5sum is installed on the usual Linux installation. If you do not yet have Linux installed, you can run md5sum on Windows as well. Two programs for this are: winMd5Sum or Md5summer.

It is easy to install and has a nice GUI. Navigate to the file in question and run the md5sum. It has a compare line where you copy and paste the checksum and click on compare to see if they are the same.