As cool as 3D printing is, there is a lot of hype around it, which sometimes causes confusion. Before starting to design for 3D printing, it's best to know a little bit about 3D printing technologies.
3D printing is a limitless technology in the sense that there is no end to the things it can make. Still, that doesn't mean that it can make anything without limitations. 3D printing can make things that no other manufacturing method can, but it has rules that need to be followed to ensure success. There are different types of 3D printing as well, and each type comes with its benefits and drawbacks:

In this chapter, we'll discuss:
What is 3D printing?
What types of 3D printing are there?
How do FFF printers work?
The anatomy of an FFF print.
Supportless 3D printing and YHT.
Wall thickness and tolerances.
3D printing is cool. It seems as if not a day passes without another mention of 3D printing online in the news and media. Everyone is getting excited about 3D printing. But when you look deeper, it seems as if everything is being 3D printed, and anything could be. Does 3D printing something make it better? What exactly is 3D printing?
In many ways, 3D printers are just tools, the same as any that you'd find in a wood shop or garage. These tools make cool things, but not on their own, and just because something is made with, say, an electric drill press, that doesn't automatically make it better than something that isn't. It's the things that people, like you, are doing with these tools that make them cool.
I'm not saying that 3D printing isn't cool by itself. 3D printing lets you create things, test them, change their design, and try something new quickly until you get it right. It makes things of incredible complexity and, because it's additive manufacturing, generates comparatively little waste. The availability of cheaper and faster 3D printers means that there's a chance that there's a 3D printer near you.
There are many different types of 3D printers, but what makes them all similar is that they build solid shapes from layers of materials, starting with an empty build area and filling it with the print. This is called additive manufacturing, and it produces less waste than other techniques, such as starting with a base material that is cut away to make the thing.
3D printers also benefit from being computer-controlled machines, also known as computerized numerical control (CNC) machines, meaning they do what they do with minimal human interaction after the design work is done. They can make many identical copies of a thing one right after the other, and the design can be shared online so that others can make their own copies.
While all 3D printing shares come common features, there are several distinct types of 3D printing that vary in how they produce the print. Fused filament fabrication (FFF), powder bed, or light polymerization, for example, all accomplish 3D printing in very different ways, and each with their own strengths and weaknesses. What works in powder bed 3D printing might not work with FFF 3D printing, and the part you get from light polymerization might not be suitable for the same usage as those made with the other techniques.
It is the best practice to always design towards the strengths and weaknesses of the medium you'll be using. The projects in this series of books will focus on designing for FFF 3D printers, because they're inexpensive and more readily available than the others, and the parts made with FFF 3D printers are suitable for a wide variety of functional uses. Also, many of the techniques for FFF design transfer to the other types of 3D printing. But because FFF 3D printers have limitations, there will be some things you need to know first.