Copy/Paste from Sweetwater website:
The main difference between a workstation and an arranger is that the arranger keyboard (LIKE YAMAHA PSR-SX900) has built-in “auto-accompaniment.” In other words, with the push of just a few buttons, the keyboard will provide backing tracks that automatically match the style, rhythm, and tempo of what you’re playing. This has become a unique tool for composers who have to produce music quickly in a style they may not be familiar with. Even songwriters who don’t know how to program drums or play guitar can use the arranger to hear their songs being played by a band, without having to call in musicians. Finally, for the working solo performer, the arranger can add new dimension to your sound by giving you the advantage of sounding like a full band.
Please note that Yamaha PSR-SX900 is advanced keyboard. If you are complete newbie like I am you should go with Yamaha PSR-E363 or Yamaha PSR-E463 for intermediate user.
Hope this helps.