Like other major churches in Germany the Speyer Cathedral is called a "Dom" (dome). This has nothing to do with architectural features, although it has a dome on the eastern bit, but is a description for the hirarchically standing in the church.
It is also called Kaiser- und Mariendom (Cathedral of emperors and Mary), and it is the dominating feature in the town. It's size and importance in history is far greater than that of the town around.
The building was started in 1030 by the first Salian Emperor Konrad II and has changed quite a bit since then. Today the Speyer Cathedral is the biggest remaining romanesque church in Europe. And it is, like almost every big and old church in Germany, property of the catholic church.