SLD / Symbology Encoding
The OGC Symbology Encoding (SE) standard has initially been a part of the Styled Layer Descriptor (SLD) 1.0 standard. In august 2007 the OGC declared the separation of the standards (ext. link). Since version 1.1 the SLD standard only describes the styling-related communication with OGC webservices. The actual syntax for the description of styles got a new name: OGC Symbology Encoding 1.1 (SE).
Styled Layer Descriptor (SLD) is still widely-used as a synonym for Symbology Encoding (SE). The common filename postfix is: .sld
AtlasStyler is graphical editor that allows to create maps and export their symbology in form of .sld files.