GIS
A system for storing, analysing and mapping data that has a location attached, so you can ask where something happens and not only how much of it there is.
A geographic information system, usually shortened to GIS, is software for working with data that has a location attached. It stores that data, analyses it and draws it as a map. The defining feature is that location is part of the data rather than a label on top of it, so you can ask spatial questions: what is near this, what falls inside that, how far is it from here to there.
A GIS holds data in layers. One layer might be streets, another schools, another population by census cell. Because every layer shares a coordinate system, they can be combined, and the combination is where the answers come from. Streets plus schools plus population tells you how many children live within a ten minute walk of a school, which none of the three layers says on its own.
GIS started as desktop software that had to be installed and maintained. Much of it now runs in the browser, which is what WebGIS means.