what is the GIS ? what is the output ? who used it??

Question

Geographic Information Systems (GIS)
GIS is a system of hardware and software used for storage, retrieval, mapping, and analysis of geographic data. Practitioners also regard the total GIS as including the operating personnel and the data that go into the system. Spatial features are stored in a coordinate system (latitude/longitude, state plane, UTM, etc.), which references a particular place on the earth. Descriptive attributes in tabular form are associated with spatial features. Spatial data and associated attributes in the same coordinate system can then be layered together for mapping and analysis. GIS can be used for scientific investigations, resource management, and development planning.

GIS differs from CAD and other graphical computer applications in that all spatial data is geographically referenced to a map projection in an earth coordinate system. For the most part, spatial data can be "re-projected" from one coordinate system into another, thus data from various sources can be brought together into a common database and integrated using GIS software. Boundaries of spatial features should "register" or align properly when re-projected into the same coordinate system. Another property of a GIS database is that it has "topology," which defines the spatial relationships between features. The fundamental components of spatial data in a GIS are points, lines (arcs), and polygons. When topological relationships exist, you can perform analyses, such as modeling the flow through connecting lines in a network, combining adjacent polygons that have similar characteristics, and overlaying geographic features.

Answer 1

So what exactly is your question? You seem to have found a source that tries to define it. That's basically it.

But I will simplify:
GIS is a database of information with a geographic reference. People who use GIS use it for planning, research, utility and facility management, defense, and all sorts of applications. Basically, if you need to know where something is, and some sort of information about it, use GIS.

With regards to GIS vs CAD, CAD is a static image that doesn't retain the same amount of information and analytical processing.

If someone out there really wants to know, check out this site.

http://www.esri.com
It's like the microsoft of the GIS world. They also have industry solutions.

Answer 2

The ESRI site that Master C points to is a good source.

I think of GIS as being comprised of two major components: 1) mapping and 2) a database. What you get out of it is highly related to what you put into it. As with all databases, it is pretty much useless until it is populated somehow. Once populated, though, it can provide very useful results to your queries.

For example, pipeline operators in the US need to keep track of what are termed "high consequence areas", areas where a pipeline failure would result in large human, ecological, property, or other losses. Clearly mapping is part of the problem, so the pipeline company enters information about where their lines run. But they also enter population data, location of certain facilities such as schools, hospitals, highways, etc., locations of waterways or other sensitive areas, direction of runoff, maybe even migratory bird data.

With the information in the database and an understanding of what constitutes a high consequence area, the pipeline operator can simply ask GIS to "show me the HCAs" and the program can map out which portions of the pipeline would be classified as HCA.

GIS, like any database, requires a lot of work up front. The benefits are realized once the database has been populated.