Here's a list of the main categories most (if not all) music synthesizers fall into. When putting together a studio or live rig, it is often useful to have a selection of various keyboards to produce a wide variety of different sounds - so this list may give you a few ideas of what to consider purchasing.
Analog - Monophonic: These things have been around since the beginnings of synthesis technology – after all, they are the simplest to create bearing in mind that they need only to have the facilities to produce 1 voice, and that they use less complex (but many would argue better sounding) analogue components. Examples: MiniMoog and ARP Odyssey.
Analog – Polyphonic: A polyphonic version of the above, meaning that they are able to play more than one note without compromising the other voices. In the early days these were gargantuan beasts with a sound to match, but the Prophet 5 revolutionised analogue polyphonic synthesizers due to its use of a computer microprocessor, giving it the ability to digitally store patches. Examples: Prophet 5 and Oberheim OB-8.
Virtual Analogue: The modern day equivalent of analogue polyphonics, these digitally powered synthesizers are able to play a large number of voices, store lots of presets, and never go out of tune as they are powered by a microcomputer. First introduced in 1995 with the Access Virus, they now feature in-built effects ad sequencers to add to their power. Examples: Clavia Nord Lead and Roland JP-8000.
Digital: A relatively short-lived group of synthesizers, mostly from around the 1980s featuring alternatives to subtractive synthesis. Yamaha pioneered this idea with their DX range of FM-based synthesizers, as well as contributions from Casio with their phase distortion technique, and Roland with their still very popular D-50 synth, which used a new type of synthesis called ‘Linear Algorithmic’. These synths, although responsible for the downfall of analogue machines, helped give musicians the ability to create completely new and unique sounds never heard before. Examples: Yamaha DX-7 and Korg M1.
Hybrid: Unique machines with special characteristics, so it’s hard to describe them all together. However, they are usually defined by combining analogue and digital technology together, or for introducing a new method of synthesis or major feature. Examples: PPG Wave and Ensoniq ESQ-1.
Semi-Modular: These are a long outdated combination of analogue monophonics with the patch points of a modular synth. This has the advantage of a fairly small synthesizer but which can be programmed with great complexity. Software technology makes this sort of synth architecture no longer necessary. Examples: ARP 2600 and Korg MS-10.
Modular: Analogue synthesizers which feature each of their individual components in separate units. These units (or ‘modules’) are connected by cables using the CV/gate trigger interface, and have the advantage that they are extremely flexible in terms of programming ability, and allow the user to add or remove modules as much as they want. Unfortunately, this also makes them difficult to program and high addictive (and therefore large and expensive). Examples: Moog Modular and EMU Polyfusion.