Uday,
it's documented here as a sample: http://help.arcgis.com/en/webapi/javascript/arcgis/help/jssamples_start.htm#jssamples/util_project.h...
The Usage of the Geometry Service is documented here: http://help.arcgis.com/en/webapi/javascript/arcgis/help/jsapi_start.htm#jsapi/geometryservice.htm
A list of Coordinate Systems is also documented:
http://help.arcgis.com/en/webapi/javascript/arcgis/help/jshelp_start.htm. See "Coordinate Systems IDs".
A sample Geometry Service is here: http://sampleserver3.arcgisonline.com/ArcGIS/rest/services/Geometry/GeometryServer
Regards,
Axel