Simply put, a CAD program is a drafting board built into a computer program. But it’s even more than that! The accuracy obtained in these programs can be measured to 1/1000 of an inch, even though this can only be measured by the line thickness of the paper drawing that is eventually output by the program.
Current CAD software packages vary from two dimensional, vector-based drafting programs to three dimensional solid and surface model systems. Modern CAD packages can show views rotated in all three dimensions, allowing you to view a desired angle even from the inside of the object.
CAD is used in the following industries: Architectural (houses and buildings), Mechanical (machine and shop), Industrial (plant layout and parts manufacturing), Survey & Topographical (plats, site plans, and topography), and many other fields where accurate drawing are required.
Those that work specifically in this field are called CAD drafters or specialists.
There are quite a few tutorials and courses available over the internet where you can learn to use CAD for yourself. Go to http://draftingschool.net for more details.