What is CAD?
CAD is an abbreviation of Computer Aided Design, and simply means 3D design on a computer. It is the general term for 3D modeling. But there are two quite different types of CAD or 3D modeling: parametric 3D modeling and mesh based 3D modeling.
To understand the difference between the two types of 3D modeling, one can compare them with the 2D world. Here, many will know that you have two kinds of 2D graphics, namely vectors and raster graphics - what we know as images. An image (raster graphics) consists of a lot of pixels that form an image. If you zoom in on what looks like a circle in an image, you will be able to see the pixels and that it is not really a perfect circle. With vector graphics, on the other hand, you can create a "perfect circle". Here, the circle does not consist of pixels but of a mathematical instruction to the computer to draw some curves which form a circle.
Mesh based 3D modeling
With this analogy, mesh based 3D models correspond to the raster graphics - the images. A mesh does not have "infinite resolution". If you zoom in on e.g. a 3D mesh sphere, you will see that it consists of thousands or millions of points (vertices), edges and faces. Depending on what the 3D model is to be used for, you will then adjust the resolution to not be able to see the individual faces.
Parametric 3D modeling
With the same analogy, parametric 3D modeling corresponds to vector graphics. If one has drawn a cylinder in parametric CAD, it will actually be a cylinder with "infinite resolution".
Use:
Mesh-based 3D models are used for many things. They are, for instance, used for CGI in movies and for computer games.
Parametric 3D models are typically used in engineering, construction and production – CNC machines etc. However, there is one exception where mesh based 3D models are used for production: 3D printing.
Modeling:
Because the two types of 3D modeling are so different, they is also quite different to work with. The two types of 3D modeling software are similar on the surface, because there are typically a 3D view of the work and some tools around it. But where parametric 3D modeling software will have tools to manipulate the mathematical "instructions" of the 3D model, there will be tools to manipulate the individual vertices in mesh modeling software.
File types
The most used file types for mesh based 3D modeling (and therefore also 3D printing) are:
STL (the simplest)
OBJ
FBX
X3D
In the parametric world, the file types STEP/STP or DWG are the most used, but often it will be the filetype native to the CAD software that will be used.
Back and forth
Fortunately, it is easy to go from a parametric 3D model to a mesh based 3D model that one needs for 3D printing. In most CAD software there will be a button for exporting to e.g. STL, which is often used for 3D printing. It is somewhat more difficult to go from a mesh-based 3D model to a parametric CAD model, however. It is hard for the software to guess what are the actual edges and what are the edges are part of a surface. Therefore, one is often forced to re-make 3D models from scratch if they are needed as parametric 3D models. Sometimes, however, one can convert a mesh-based 3D model directly to a parametric 3D model relatively easily, especially if the 3D model is made with that in mind.