The sound module is generally a sample-playback synthesizer. The concept of a sequencer combined with a synthesizer is not entirely new – the Sequential Circuits Six-Trak provided this already in a crude form. All parts of it were purely based on subtractive synthesis; so no preset drum kits, a thing every sample-playback synthesizer since the Roland D-50 featured.
However, the incarnation of the idea reached its maturity (and a boom in sales in the truest sense – 250,000 sold, the most a synthesizer ever did!) with the Korg M1. Besides just a sequencer, it also provided a large enough display, a vast array of sounds (with the woody Piano sound and the "Universe" patch being the most famous), and built-in effects. Floppy disk drives were included on later machines, making it easy to store the sequencing data (either as proprietary or Standard MIDI File format)
Nowadays, workstations have evolved to the point that they can either include a DSP-based synthesizer upgrade (Korg MOSS board for Trinity and Triton workstations, Yamaha AN-PLG and DX-PLG plugin boards), more multisamples and preset-memory locations(Roland JV/XP and SRV/XV series expansion boards, Korg EXB-PCM expansion boards, various Yamaha PLG-boards) and even a complete sampler (Korg sampling expansion for the Trinity) or a possibility for treating audio via the external inputs (Yamaha VH-PLG plugin board).
The Big Three (Yamaha, Roland and Korg) have sampling now as a default option in respectively the Motif (ES), the Fantom (S/X) and the Triton. They have a relatively big screen to give a comprehensive overview of the sound, sequencer and sampling options (since the display is one of the most expensive components of these workstations, Roland and Yamaha chose to cut back by not using a touch screen display, and in case of Yamaha not even a high-resolution display). The screen replaces what would otherwise be a lot of extra rotary knobs, sliders and buttons, which add a lot to the cost of the machine, make the operation look unecessarily complex, and generally aren't used in the first place.
The sequencer stores events like notes and controllers (like pitch bend), and then replays them into the sound generator, which then makes the music.
Although many music workstations have a keyboard, this is not always the case. In the 1990s, Yamaha, and then Roland, released a series of portable music workstations (starting with the Yamaha QY10). These are sometimes called Walkstations.
The concept of the workstation mutated around 1996 and gave birth to the groovebox – a key-less version of a workstation, still with a self-contained sound source and sequencer, mostly aimed at dance. Again, nowadays they also feature a sampler. Roland more or less started the hype, and Yamaha, Korg and E-mu followed suit.