When it comes to raster data, Web Coverage Service (WCS) is the equivalent of WFS for delivering original data. Like vector data, raster data may be rendered in a proper way on a map, and you will get the result with WMS and a GetMap request. WCS is intended to get a raster dataset or its subset in its original form, without any rendering or other processing and data transformation.
With WCS, you don't have a standard format for data delivery; it depends on the original format of your data.
The current release of GeoServer supports the 1.0.0, 1.1.0, 1.1.1, and 2.0.1 WCS versions.
As with WFS, you can find the full reference for WCS at the OGC repository (http://www.opengeospatial.org/standards/is); look for OpenGIS Web Coverage Service (WCS) Implementation Specification.
