WMS stands for web map services. This is one very common mechanism to
show geospatial content. It is a standard recongized by the Open Geospatial
Consortium.
Usually, you can find WMS services from government agencies, universities, and other
entities.
WMS services are usually free to use. WMS services can be produced by a variety of
software,
the most common are ArcGIS for Server, GeoServer, and THREDDS.
Web applications use common JS libraries such as Leaflet, OpenLayers and others to
display WMS content.