VST (Virtual Studio Technology) is the standard plugin format for digital audio. Plugins come in two types: effects (VST effects) that process audio — EQs, compressors, reverbs, delays — and instruments (VSTi) that generate sound — synthesizers, samplers, drum machines, orchestral instruments.
Modern plugins range from free to thousands of dollars. Universal Audio's UAD plugins emulate vintage analog hardware (Neve preamps, LA-2A compressors) and require UAD hardware (DSP chip in Apollo interfaces) or run natively. Waves, FabFilter, iZotope, and Valhalla are other major plugin developers. Many DAWs include a solid collection of stock plugins — you do not need to buy plugins immediately.
The plugin format varies by platform: VST (universal, Windows/macOS), AU (Audio Units, macOS only), and AAX (Pro Tools format). Most plugin developers release all formats, but always check compatibility before purchasing.