Unless he is talking about the different rating systems. Of which, that is a different story.
EDIT: My bad, he did say "systems." I will answer your question to the best of my ability, kanecdc. If, perhaps, you weren't talking about the different systems, such as USCF and ELO, then this will be a fail of a post and I will apologize ahead of time if such be the case. Anyways, back to the explaining:
There are different "systems" of which chess players are rated. These systems are used by different organizations across the globe.
If you want to see about rating systems, go here.
Why the hell are there three different rating systems? I have a rating of 1130, 1252, and 1363. I am only one player, and I only need on rating. So why is there no corilation between the ratings?