What is Parametric? and what is the use of it in AutoCAD?

see the detail article  http://www.mycadsite.com/tutorials/level_2/2-9.htm

Parametric constraints allow you to force an object to behave the way you want it to. If you need a line to remain vertical at all times, you can set a constraint on it to do just that. Need2 circles to remain the same diameter? How about a circle for a bolt that must stay1/2" diameter? No problem with constraints. These are just some examples that will make it easier to control your drawing.

Constraints can be divided into two groups and are on the Parametric Tool Panel:

Geometric Constraints

Constrains a object based on geometric properties : vertical, horizontal, etc

Dimensional Constraints

Constrains an object based on a set length or radius.

Mr. Shakeel has a good post on this subject..........

Mr.Shakeel an excellent answer and do not need Add...thank you

I agree with other specialties. 

Parametric (rule-based) drawing is by far the best way to enforce design intent in2D drafting. Design intent in AutoCAD (or in any other engineering software) means that when drawings are edited to make “this part” wider and “that hole” larger, all the attached or related objects behave in a predictable way that honors the designer’s intentions in creating the drawing.

Thanks to experts 

Well answers