Arranger: A keyboard with the ability to play styles/auto-accompaniment usually in the leftmost side of the keyboard. It may or may not also be able to record tracks on the unit.

Workstation: A keyboard that has the ability to record MIDI tracks onboard the unit (no styles).
Examples:
Yamaha PSR2000: Arranger with MIDI recording
Yamaha Motif: Workstation (no Styles)
Yamaha S90: Performance keyboard (No styles - no MIDI recording on board)
Starkeeper