MapServer is an open source development environment for building spatially-enabled internet applications. It can run as a CGI program or via MapScript which supports several programming languages. MapServer was developed by the University of Minnesota — so, it is often and more specifically referred as “UMN MapServer”, to distinguish it from commercial “map server”.
MapServer supports OGC services such as WMS, WFS etc. to serve geospatial data over the web.