What is (Mechanical Engineering ) CAD software?

This description is for general people. I am starting my description with very simple example so that any person can understand about CAD softwares.

All of us know what is a rectangle and how to create it, since our school days. But see now, how easily a rectangle can be created in CAD software that is used by a CAD designers on the following
tutorial.

Simple as well as complex geometric shapes created in 2D can be converted in 3D using a CAD application. See the following most simple part creation examples:-

There are many software in this field. I have used Autodesk Inventor, SolidWorks , Siemens NX and Creo Parametric my self that is why I recommend them for such work.
The models are created on computer before manufacturing, every thing is analyzed (weight, material, mass, cost etc). The models can be modified at any moment . Then the final output is manufactured by factory using CAM software.
There are CAD (Computer Aided Designing) software which witch are integrated with CAM (Computer Aided Manufacturing) software.
To know more on this subject please visit the following link on Wikipedia.
http://en.wikipedia.org/wiki/Computer-aided_design