Plug-ins and libraries are extra features that can be added on top of the built-in features of Audacity. Plug-ins can make special sound effects, analyze audio content, or just add to the long list of effects already available in Audacity. Plug-ins are typically used by advanced users of Audacity, or those users who want to add an additional effect to their audio track that can't be found among the pre-installed options.
The most common plug-ins include:
Nyquist plug-ins for audio synthesis and analysis.
LADSPA effect-based plug-ins that can be plugged into a wide range of audio synthesis and recording packages, like Audacity.
VST effects, or Virtual Studio Technology, plug-ins use Digital Signal Processing (DSP) to simulate traditional recording studio hardware with software.
VAMP plugins that are for viewing and analyzing the descriptive contents of music audio files. This could be things like histograms or curve data.