An arranger is a keyboard with a special ability to give you rythms like drums and auto accompaniment where you can sound like a full band with a few pushes of the fingers.
I believe this is one of the best way to learn music and a great tool for composing also.

It's easier when someone demo it to you in live. You can click the link below and see the different keyboards being demoed by a real musician (Michael V is famous around here).
http://www.yamaha-europe.com/yamaha_euro...ovies/index.php

You can see the PSR3000 or the much cheaper PSR295 and PSRK1 in action.