Mapserver is an open source package distributed via the university of minnesota.
It is one of a number of tools that can use postgis data, and it handles a wide variety of other formats aside from postgis, including Web Feature Servers.
Mapserver is – like postgis – not a desktop GIS system. Primarily, it functions as a cgi tool for web pages, allowing interactive clickable maps to be created. However, it can also be used via a programming interface, which is how we use it.