A common map contains more than a layer, each one styled with one or more symbols according to its complexity and the map purpose. How can you create a multilayer document with SLD? You can't. As the acronym states, an SLD document can contain a rule relative to just one layer.
By publishing your layers with one or more styles associated on GeoServer, you can compose a map with an external client supporting a WMS protocol, for example, an OpenLayers JavaScript client or a desktop GIS such as QGIS.
Another possibility offered by GeoServer is the layer group. A layer group is a set of layers with a drawing order. Using layer groups, you can compose and publish a full map as if it was a single layer. Your client will have to do a single WMS request to get all the layers.