Explanation
Styled Layer Descriptors (SLD), are what make your maps colorful. They tell the server how the map should be rendered, whether to draw lines in black, or to color them in blue with a nice outline and text labels.
SLD is an XML based language detailed in an open specification available here. It is used for rendering by many OGC compliant Web Map Servers (WMS) and Web Feature Server (WFS) clients (like Mapbuilder).
There is a good SLD Tutorial here.
