Parametric* Constraints are not new in CAD but are in AutoCAD 2010. I think their arrival, along with free form modelling, reflects a real change in attitude from Autodesk towards the application that started it all.
AutoCAD is now termed as "the platform which democratises design" and feature sets previously only seen in verticals, or even other platforms, are starting to appear in AutoCAD. It also goes the other way as the Ribbon UX pioneered in AutoCAD is appearing in other Autodesk applications. I'll be writing more on these topics in future posts but it sets the scene for the arrival of a feature I thought "plain" AutoCAD would never have. Before the question is asked: AutoCAD LT 2010 will honour parametric features in existing files (you can view, edit or delete) but not create them.
Currently AutoCAD parametrics apply to 2D objects, although you can fudge limited manipulation of 3D objects. There are Geometric Constraints which control object spatial relationships and Dimensional Constraints to control size. If you are familiar with Inventor the parametric tools in AutoCAD will seem similar. If not, learning AutoCAD Parametrics should make transition easier should you require it in the future.
Parametric Interface:
Parametrics have a dedicated Ribbon Tab which groups the tools and their visibility controls in a logical arrangement. A new Parametrics Manager Palette, launched from the ribbon, allows tabular editing of names, formulas and values once dimensional parameters are applied. (click thumbnails to enlarge)
Why Parametric?
The applications of Parametrics are limitless but one simple example shows how they can change the behaviour of an AutoCAD object. Below you see two objects created with the veteran 
Rectangle tool. It's always been a bit of a fib as just creates a simple 4 node closed Polyline in the shape of a rectangle. Move a corner grip, as seen on the left, and you find your "rectangle" rapidly becomes rather un-rectangular! On the right, the same geometry has constraints applied to keep it rectangular as the corners are grip edited.
The combination of parallel, perpendicular and horizontal constraints is indicated by control icons you can toggle on/off or use to edit the constraint. As sharing files with parametrics becomes more common a good first step will be to toggle on the display of these indicators to check the configuration.
Applying Parametric Constraints:
The combination of constraints on the "rectangle" above was added with a simple click of the Auto Constrain Tool. It applies geometric constraints to a selection set of objects based on their relative orientation.
Some, most?, parametric applications require an object to be fully constrained but AutoCAD does not. Below I've added a perpendicular constraint to two lines without any dimensional constraints. These lines remain perpendicular as their length or location changes. This is a very flexible approach to parametrics but one which will require some thought in it's application.
Geometric constraints: These include controls for Coincident (with other object points), Lock (to an absolute location), force Horizontal/Vertical, Parallel/Perpendicular/Colinear, Concentric/Tangent for arcs/circles. Others include Equal to have geometry follow its master (e.g circle diameters which match), Smooth will join splines and Symmetric matches characteristics about an axis. When adding these the first object selected becomes the master,subsequent elections follow it..
Dimensional constraints allow direct entry distance/angle to alter object geometry or formulas referencing other parameters.
The rectangle below has a direct entered length parameter, d1=7, and a formula, d1/2, for the height. Alter d1 and the height will adjust to follow. The process of adding constraints is very similar to adding the dimensions they resemble. Names d1,2...etc are assigned automatically but can be edited afterwards if you wish. If constrained objects are copied new parameters are created for the new object. A copy of this rectangle would have d3=7 and d4=d3/2.
You can edit dimensional constraints in the Parametrics Palette or drawing editor. This can be done by entering a value or formula into the "control dimension" or by selecting the dimension and using the grip controls to drag.
You can also "relax", temporarily over-ride, constraints by cycling Control while editing. AutoCAD will advise of possible impacts/conflicts with other constraints and options to resolve them before completing the edit.
This is a quick introduction to a powerful new tool-set. I hope it gives some idea of the work flow and interface. Look for more complex examples in future posts.
* Few will understand how hard it was not to type paramedic constraints while composing this...
