A comp track is a single "best of" track that's put together from multiple takes of the same song. To assemble it, listen to the individual vocal takes and determine where the best performances of the various lines and sections are. Do this with a lyric sheet and pencil in hand, writing down the segments you prefer: verse one, take three; verse two, lines one through three, take two; verse two, lines four through six, take four; and so forth. Once you have your road map, you're ready to go.
On a tape-based system, bus all the tracks to a new track. Make sure the levels all match, roll tape, and "play" the comp using the channel mutes. Mute and unmute the various tracks as you go through the song to get the best performance onto the comped track. The great thing about this process is that it's completely nondestructive. If you screw up, the original takes will still be intact, and you can just try again. If you make a mistake halfway through, you don't have to start from the top; you can punch in on the comp track and pick it up from the line before your error. When you're done, listen to your newly created comp track and make sure you're happy with the performance and the levels.
If you're working on a hard-disk-based recorder, assembling a comp is even easier. I usually pick the track that is the best overall and simply cut from other tracks and replace sections in the master track. (I would recommend cloning the master track first so you have a backup in case you're unhappy with the comp.) You also have the luxury of doing subtle time shifting if you want to make rhythmic adjustments.