What is the difference between a keyboard workstation and a regular keyboard?

The main difference between workstations and other keyboards and synthesizers is the sheer amount of features and processing power. A key part of the workstation is the ability to sequence and play multiple synth and rhythm tracks internally when a regular keyboard simply doesn't possess the polyphony and multitimbrality to do so.

What should I look for in a keyboard workstation?

Some good features to look out for are multisampling and the ability to record audio, plus a good selection of "real-time" controls on the front panel.