Essentially, I could go into a history lesson, but
Wikipedia does a far better job. it also contains some useful terms.
Basically, a tracker is a piece of audio software that has a specific grid interface, which unlike most music notations, time is represented vertically. In each position of the grid, you can place notes, effects, etc...
There are several similarities in all tackers, but each tracker has it's own subtle differences. I encourage you to try more than one tracker when starting to find a feature set and interface you are comfortable with, then learn it fully. From there, when you hit a limit, try another piece of software and learn it fully too. Once you learn 2 or 3 different trackers, you will be able to use any tracker (although if you're like most of us, you'll pick one and stick to it because you are more creative in one tool than another).
