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CAD CNC SOFTWARE
In the early years of NC machinery all programming was done manually at the machine. Then cheap and powerful computers hit the market and the need for improved efficiency and accuracy in machining encouraged the use of computerized drafting to prepare drawings for part production well ahead of the production process. CAD or Computer Aided Design software added a visual and engineering component to the design process that preceded the actual computerized coding element and allowed for trouble shooting and testing BEFORE a design ever made it to the production floor. So CAD software takes your hand drawn ideas and translates them into these computer drawings, with drafting specifications and accuracy, ... as well the CAD software renders them on your computer monitor in a fashion that makes it visual obvious what you have created.
The challenge is to find software that will do what you have to do and not break the bank, not an issue if you have won the lottery lately, but most of us don't fit into that category. So first lets look at what might be the bells and whistles of CAD CNC software and then you can have a better idea on what specific capabilities might be relevant to your particular wood working applications. 2D or 3D capabilities Is typically the most critical question!.. or more correctly 2 1/2D and 3D... Hopefully we all understand that 2D drawing is what we do all day long when we add our chicken scratches to a piece of paper. We move across the page or down the page or some combination to create a shape.. In CNC terms this is traveling along the X axis (or across the page) and/or down the Y axis, as in up and down the page. To create something like a circle you need to travel incrementally in both the X and Y direction at the same time.
The most common application for this in the woodworking arena is in kitchen cabinet and door construction and basic wood signs. Full sheets of plywood, MDF or solid wood are dropped on your CNC table and the software helps to layout how exactly the sheet should be cut. The CAD software has to be dimensionally correct but not particularly sophisticated. 3D Software:
Now before someone jumps out and tells me that 3d drawing software is NOT always necessary for 3d CNC carving, I'll tell you that that is quite true. There are other methods to create 3d tool paths without having the challenge of creating a 3d pattern in CAD software, but we'll get to that a little later on. 2D or 3D CAD CNC Software:For now in a discussion of CAD remember its purpose is to convert your line drawings into digital electronic representations that can then be used in the next step to automate your cutting process. Your decision is whether all the cutting you intend to do with be in outline mode or require more sophisticated 3d modeling. The software you buy today must match your production requirements, and 2d vs 3d is the first BIGGIE consideration... but be forewarned that the learning curve for 3d modeling is pretty steep, so staff expertise may be a bigger consideration than the software.. but that's a story for another day. Also realize that you can always upgrade. Once you become more experienced at CNC woodworking, you will want to expand your capabilities. If you are a hobbyist or small woodworking business and not really quite sure yet how exactly you intend to use this technology, I'd buy cheap 2D software and then as your electronic drawing skills and CNC machining skills improve consider purchasing more sophisticated software to expand your capabilities. At that point you'll have a better idea on what you want to do and what your machine is capable of doing. Some examples of CAD software might be goggle's Sketch-up, Traditional graphic arts software like Adobe Illustrator or the originator of industrial CAD software: AUTOCAD Lets continue this conversation by discussing other features of CAD CNC software that might be significant to your implementation.
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2 1/2D is when your machine will create 2 dimensional cutouts while dropping the head of the CNC router down to a specific depth, typically BEFORE it starts cutting. This is the basic functionality of all CNC routers on the market today. From a CAD CNC software perspective its all pretty easy. All you need is 2 dimensional drawing capabilities to start with and the CAM software which we'll talk about next will allow you to define depth of cut as one of its parameters when you create the tool path. 
3D is taking CAD software to a totally different level of sophistication. Now it has to not only prepare outline cuts but add surface profiling to the drawing. Think of the fancy carving on the corner of cabriole legs, or engravings added to fancy wood signs. Each of these applications requires a 3 dimensional drawing to be prepared before the G-Code can be created to execute the appropriate tool paths. 
This has been quite common in the metal working industry where components are milled out in various shapes and configurations such as in the drawing to the right. In the woodworking industry it is relatively new, adding creative features to traditional flat panels or plain legs. Have a look at the corner of the leg just above.