The Style Layer Descriptor is an encoding for how the Web Map Server (WMS 1.0 & WMS 1.1) specification can be extended to allow user-defined symbolization of feature data.
- SLD 1.0: 2002-08-19 http://www.opengis.org/docs/02-070.pdf
SLD can also refer to the XML document which describes how to draw specific features. Eg, A road line may be drawn in red, 3 pixels wide.
One of the nicest ways to learn about SLD is using the Geotools Style class and tutorial.