In its simplest form, web GIS can be defined as any GIS that uses web technology to communicate between a server and a client (Esri)
A hot buzz term that you’ll be hearing quite often these days is “Web GIS”, however, you may be wondering to yourself, what is Web GIS as it seems fairly broad in scope. According to Esri, Web GIS is a type of distributed information system, comprising at least a server and a client, where the server is a GIS server and the client is a web browser, desktop application, or mobile application. Web GIS offers many advantages including ease of use, support for unified updates, diverse apps and more.
From SSP – The term WebGIS is being tossed around all over the place right now but the true meaning of the term may be very different than what you understand it to be. This video covers an explanation of how Esri WebGIS provides for the evolution of your GIS from a System of Record into a System of Engagement. WebGIS is more than maps on the web; Find out how you can utilize an Esri portal to transform your entire GIS roadmap.
Read more on this here as well as a description of the 5 key elements and a good video from SSP describing Web GIS