A software synthesizer is a PC computer program that generates music and synthetic sounds. For a few hundred dollars, a software emulation of a hardware synthesizer can replace the expense of having dedicated synthesizer hardware. This allows great flexability, but some aficionados claim that the sound from a software synth is not as good as the real thing (in hardware). Also, a PC user interface is not really suitable for live music performance. The user interface problem is solved with a wide array of MIDI keyboards and controllers, that plug into the PC to drive the PC synth software.
Cherry Audio - ARP quadra SW synthesizer, $39 bucksThis software synth will work with either with DAW for this ARP Quadra plug-in, or use this plugin in stand alone mode. You still need some kind of MIDI controller. Supports many software backplane buses, like AAX, AU, VST, VST3, and Standalone.
The options for expanding the software synth voices and effects can be met with the VST plugin software backplane. Thousands of plugins exist, both commercial and freeware, and many audio applications support VST interconnection of software modules. There are many other software interface standards, VST may just be one of the most popular.