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
other geospatial content.

Starting on v1.4, it can open local vector files (KML, GPX, etc, on EPSG:4326).

Relevant features:
- feed web services URLs and Map Services Visualizer will try to figure out which services and layers are being served at the endpoint
- set the layer order (which layer is over which), transparency and visibility of each one through controls on TOC (Table Of Contents)
- with GPS on, it will show a marker of you current position. You can then set to keep your position in the center of screen and even to rotate the map based on your heading, useful when moving around with your device

Free version has ads and lets you open a single web service at a time.

Premium version is ad free, removes the cap of one service only and saves the Table of Contents configuration between app restarts.