SLD / SE
If you are interested in Symbology Encoding (SE) please read HowToSE.
If you want to know how to use Styled Layer Descriptor (SLD) in a WMS GetMap request read HowToUseWMSGetMapRequestsWithSLD.
You should also take a look at the OGC specifications for:
Users interested in a quick reference of useful filter encoding functions to be used in styling, have a look at the filter functions overview page.