Self Timer - Just like any self timer on your camera. Except where most Canon cameras offer either a 2 second delay or a 10 second delay, the TC-80N3 allows you to set any delay (in 1 second increments) up to 99 hours, 99 minutes and 99 seconds - That's just over 4 days!
Interval Timer - The Interval Timer can be set to any time period up to 100 hours as well. If you set it to 10 minutes, for example, one exposure will be taken every 10 minutes until either the film runs out or the Exposure Count limit has been reached..or the batteries die! (though that's unlikely with a 20D!)
Long Exposure - This one is easy. It allows you to take time exposures up to 100 hours in length
Exposure Count - This setting permits you to set the number of exposures that will be taken, up to a total 99. If you set it to 3 exposures you can combine it with an EOS body's auto-bracketing capability