Fairly recently, Sony came up with an elaborate way of preventing consumers from being able to copy the audio from popular CD releases to a computer. Included on each copy protected CD is a nonsensical data track that throws the computer off, and will not let it read the audio files from the CD. The recent release of Radiohead's Hail to the Thief left a lot of non-American or non-UK people wondering how they could rip their new CD to mp3 format (from what I've heard, countries like Canada and Australia have copy protection on their CDs. I have).
All you really need is a marker. Preferrably not a permanent one.
Data tracks are typically the last track on a compact disc. They are, nine times out of ten, on the thin little rim that's not quite translucent on the shiny side of the CD. All you need to do is black that part out with a magic marker. Carefully go around the edge with the marker, and try not to slip onto the rest of the CD. That's why it's good to use a non-permanent marker, so you can just wipe away any mistake you have made.
You can also use masking tape, but that's trickier and often messier. I'm told that masking tape can also be somewhat weighted, and result in a wobbly CD that does not get read properly. Using a marker is just easier.