Electronic Keyboard, Workstation, or Synthesizers...what is the difference?

For years, I ave been walking around with musical notes and eclectic sounds running through my head. I always wanted to hear those sounds outside of my mind. I kept checking out different types of keyboards with little success. What am I looking for? I want to lay down anywhere from 8-10 layers of sound and music. Which item would be the best for me? I can only spend around $600.00 or so. Please help me. Thanks, Jim