For example, there are plugins for effects boxes, such as reverbs and delays, effects pedals, like guitar distortion, flange and chorus, and for mixing and mastering processors such as compressors, limiters, exciters, sub bass enhancers, stereo imagers and many more.
DirectX plug-ins are based on Microsoft's Component Object Model (COM) which allows plugins to be recognised and used by other applications via common interfaces.
Effect plugins are used to generate, process, receive, or otherwise manipulate streams of audio.
Instrument plugins are MIDI controllable DirectX plugins, generally used to synthesize sound or playback sampled audio using virtual synthesizers, samplers or drum machines.
[1] DirectX plugins are superseded by DMO-based signal processing filters and more recently, by Media Foundation Transforms.