WPS - Web Processing Service
GeoServer's Web Feature Service (WFS) can return subsets of data but cannot return any changed data. A WPS on the other hand, can modify returned data. A WPS could perform buffer, reproject, simplify, etc.
Just as WMS, WFS and WCS have a GetCapabilities, WPS has one as well describing the service. The list of functions is available (such as buffer) in the GetCapabilities response.
Examples (many more than these are available, this is just a sample):
Aggregate - Count, Average, Max, Median, Min, StdDev, or Sum on an attribute
Buffer (Polygon - Quadrant Segments)