GeoJSON is an Open data format for describing spatial objects (points, multipoints, lines, multilines, polygons, multipolygons, etc) It is commonly used with a range of web services such as REST.
As a format is is rapidly gaining support in the Open Geo community and can be used natively in libraries such as OpenLayers and Leaflet as well as GIS applications such as QGIS and FME and also the PostGIS spatial database.
The specification is here: http://geojson.org/geojson-spec.html
As an aside, it is similar to ESRI's JSON format, but obviously has no proprietary bits and pieces. Support for Open Standards like GeoJSON would be good PR for Pitney Bowes Software, IMHO.