Using a web map server, you can build a map using layers from several different sources. Suppose, however, that both the water bodies and roads are portrayed in the default colour of black, and you would like to depict the water features as blue.

You can do this by adding a Styled Layer Descriptor (SLD) to your web map server. The styled layer descriptor is a companion specification to the web map server interface specification. An SLD is an XML document that defines the manner in which WMS layers are portrayed, or styled.

The SLD is an optional feature. A WMS must be SLD-enabled for a client to request maps using SLD. For WMS servers that do support the styled layer descriptor, there are two methods of specification:

1. Server-side; or
2. Client-side.

You can implement an SLD for server-side styling using an extension of the WMS GetMap request. The layers parameter of the request provides a list of layers to style; the styles parameter of the request provides the corresponding (requested) styles for those layers.

For client-side styling, you can either specify the SLD as a URL in the SLD parameter of the GetMap request, or provide the contents of the SLD document in the GetMap URL by specifying it as the contents for the SLD_BODY parameter. Clients can create multiple styles with multiple rule sets, filters and symbolizers.