"The shapefile format is a 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." (Wikipedia)

Mandatory files: (after Wikipedia, see the LoC and other links below for an exhaustive listing)

• .shp — shape format; the feature geometry itself {content-type: x-gis/x-shapefile}
• .shx — shape index format; a positional index of the feature geometry to allow seeking forwards and backwards quickly {content-type: x-gis/x-shapefile}
• .dbf — attribute format; columnar attributes for each shape, in dBase IV format {content-type: application/octet-stream OR text/plain}

Note also that newer media types have been requested, respectively application/vnd.shp, /vnd.shx, and /vnd.dbf (see below).

See the following for more information:

• DANS info
• Wikipedia entry
• "What is a shapefile?" from ESRI
• technical description by ESRI
• IANA vnd.shp and others registration