Short description of the Shapefile format
The shapefile format is a popular geospatial vector data format for geographic information system (GIS) software. It is developed and regulated by Esri as a (mostly) open specification for data interoperability among Esri and other GIS software products. The shapefile format can spatially describe vector features: points, lines, and polygons, representing, for example, water wells, rivers, and lakes. Each item usually has attributes that describe it, such as name or temperature.
The Role of Shapefile in SDDI
Currently, shapefiles are not being used in the SSD project. However, it can be used as one of the output formats of a Web Feature Service and can be supported by a variety of implementations.
Format Specification
Specification documents: http://www.esri.com/library/whitepapers/pdfs/shapefile.pdf