There are three main synthesizer types. They are:

Analog synthesizers

Digital synthesizers

Software synthesizers

Each synthesizer is classified according to the method of sound creation. Synthesizers that use a mixture of all these three are also available in modern market. These types are called hybrid synthesizers. Modern synthesizers vary significantly in their working method, potential, dimension and outlook. In the majority of the synthesizers the output is required to be passed through the speaker, or an external amplifier and loudspeakers to convert it into sound.

Format and Types of Synthesizers Designs.

Analog Soft Synths:

When a harmonically rich waveform (Saw, Rectangle, Square, and Triangle) is generated by an oscillator, then the filters or cuts away part of the sound is outputted in a thorough fashion with filters and envelopes. This is the mechanism of Soft Synths.

FM Synths:

Frequency Modulation works in a way where instead of subtracting from a large waveform, the synthesis is started by using several Sine Waves (operators and carriers) at user selectable frequencies and then patched them in various ways to come up with tones.

Apart from these there are others like:

Wavetable and Linear Algorithmic Synths

Sample Playback Soft Synths

Hybrid Physical Modeling Soft Synths

Hybrid and Abstract Synths

Emulator is one common type. There used to be quite a variety but manufacturers have stopped making them in a lot. Purchasing old and second hand stuff is a viable option as the unused first hand products can be expensive and unpredictable. The emulation or the copy exact types offer having graphics that model the exact user interface of controls. Popular synthesizers, for example, the Minimoog and Yamaha DX 7 have emulators.

Some software synthesizers are more of sample based. Computers, generally, have fewer restrictions on its memory or RAM than dedicated hardware synthesizers. Some of these sample based synthesizers come with sample libraries, which can weight up to GBs or giga bytes. Some of these are exclusively drafted to imitate real world instruments such as pianos. Many sample libraries are available in a Windows, IBM common format like WAV or Waveform audio format files, and can be worked with almost any sampler based softsynth.

The newer and recent versions of softsynths are designed to be compatible with technology such as Virtual Studio Technology (VST), which allow other music programs to interface with the softsynth. Plug in technologies include DXi, VST, Motu Audio System (MAS), Real Time AudioSuite (RTAS), Audio Units (AU).

Common Types of Synthesizers Designs

Most of the professional software synthesizers come with a high price tag, but given the works they can put up in real time or 1.5x real time with good hardware. These prices can be termed justified. Reality(TM) Professional Music Synthesis Software, VirSyn Software Synthesizer, Reaktor, Generator are to name a few mid range Software Synthesizers. However some free to try and freeware are out there also. Adder, Crusher X Live 3.42, Drums, Digitonix Element, Ethereal, FlexiMusic Composer, FlexiMusic Generator, SequBeat, Space Station Pro are also there. There are a number of Demos or Demonstration programs available in the market. The manufacturers give the users chance to get acquainted with the programs before they try.