From FAQ http://download.linuxtag.org/knoppix...IX-FAQ-EN.txt:

Q: What are these strange MD5 files that accompany the ISO CD images?

A: The files with the .md5 extension contain checksums for the actual CD
images. A checksum is a "matching number" for checking files. One can
verify that the file matches its original down to the smallest detail.
When you download both the ISO image for the CD and its corresponding .md5
file, you can verify that your download was complete and that the contents
are unchanged by typing "md5sum -c filename.md5" under Linux/Unix. Further
information about MD5 and programs for various OS's can be found at
http://www.fourmilab.ch/md5/. You can download a Windows(TM) version at
http://www.toast442.org/md5gui.shtml.

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