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Picture of A short, basic introduction to CAD
A brief history about CAD

In the 60s Ivan E. Sutherland (MIT) invented and introduced the Sketchpad, the early ancestor of Computer Aided Design (CAD). It featured drafting functionalities like a 2D-CAD system and it was equipped with a touchscreen as output device, a lightpen, tool buttons as input device.  
Here is a short demonstration about Sketchpad by Dr. Alan Kay: 
 
 

Step 1: Basics and which CAD software is the right one for you?

Picture of Basics and which CAD software is the right one for you?
Every CAD Software has a drafting screen, measuring-, drafting tools, layer- and blocklists.  With CAD you have the ability to make precise and detailed drafts. The price range goes from $0 to thousands of dollars.
There are several varieties of CAD programs, so which is the right one for you?
Most of the CAD-Software developers offer a demo or trial version, so you can try it. The next question is, what functions you need.
It depends on in which sector you are specialized. For example a mechanical engineer needs a CAD Software, that has the ability to visualize in 3D and simulate the behaviour of mechanical parts. A furniture draftsman requires a CAD Tool with basic functions.