The master catalog is a user catalog, but it is special because it is the catalog in which the operating system looks first (by default) when finding catalogued datasets.
The only thing that makes it the master catalog is that its name is stored in a special place that is read at IPL time. In current releases of z/OS, that place is the LOADxx member of the IPLPARM dataset used for the current IPL.
A master catalog of one system can be a mere user catalog from the point of view of another system. This is necessary in order to be able to create and maintain the master catalog in the first place.
Normally, the master catalog has all the other user catalogs connected to it, so that they can be found easily.