A map service is a server that make maps available to the web. They are usually shared using standard OGC protocols and any client that understands such protocols can read and show the service content. There are several protocols to deal with different map content, like Web Map Service (WMS) to show rasters/imagery, or Web Feature Service (WFS) to show vector.

This app purpose is to visualize the content of most OGC
services, mainly WMS and WFS. It can read, though, plenty