Shapefile spatial data format (used by many GIS programs)
SHP is a file extension for a Shapefile shape format used in geographical information systems (GIS) software. SHP is short for "shape."
GIS is a computer-based technology composed of hardware, software and data that is used to capture, display and analyze spatial data information.
The SHP file format was developed by ESRI (Environmental Systems Research Institute) as a predominantly open specification to promote interoperability between its products and other GIS software products. SHP files open in many GIS programs.
Other Shapefile file formats include the SHX file extension for spatial index files and the DBF attribute format. The SHX and DBF files must accompany SHP files to complement their information.
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Wikipedia has an entry about Shapefiles.
For a detailed description of SHP files, see ESRI's technical document.