Any map contains a set of layers, for example, a graphical representation of spatial features. Each layer contains features of a determined geometry type. When you ask GeoServer for a map, for instance, by issuing a WMS GetMap request, it extracts features from the repository (a PostGIS database or a shapefile) and draws them according to some rules. Of course, it needs a repository to store those rules; therefore, GeoServer developers needed to decide a format for the storage medium containing rules.
Map rendering is not just a GeoServer problem; unsurprisingly, it is common to all software-producing maps. Hence, it is not surprising that someone has defined a standard approach to styling layers. Indeed, GeoServer does not use a custom format for styles; instead, it leverages on an OGC standard.
The standard describes the structure of the documents and what kind of rules can be used and mixed inside the style. A document containing symbols' definitions...