3D printing is one of many manufacturing processes that fall under the scope of additive manufacturing. This is in contrast to the traditional 'subtractive manufacturing' which is essentially taking a heap of material and cutting out what you really want out of it, which inherently produces a lot of waste thereby making the process more expensive. 3D printing reduces much of the waste involved in manufacturing as well as opening possibilities for objects that are very difficult to design by hand.
How it works is some engineer or computer tech designs a custom object using Computer-Aided Drafting (CAD) software, then sends that file to a company that specializes in using 3D printers to make custom objects. The company feeds that file into a computer machine that reads the coding in the file and literally prints the object. These objects have near-limitless potential for shapes, size, and internal structures. These 3D printers can print various materials such as Aluminum, a wide range of plastics, and other metals.
A startup company in India just set the record as the first company to 3D print a rocket engine and successfully send it into low-earth orbit. I wager to say that 3D printed rockets will be the foundation of future space travel; the reason for this is because when you 3D print an object, you use only as much material as absolutely necessary, therefore cutting down on excess weight, which is essentially the #1 goal for getting things out of the gravity well. The CAD software is also capable of analyzing the structure of the object you designed and conduct stress tests, which is great for designing something that will be under stresses throughout its useful life.
Another application for additive manufacturing is literally printing human organs and human tissue. Custom made human body parts, after proven efficacy, have huge potential because they are much cheaper to produce than the current means.
Some people say that eventually most car parts can be 3D printed, and when combined with the automation & robotics industry, we're talking about a dramatic shift in worldwide labor.
I know this sounds a little ridiculously futuristic on some levels, but we are already printing everything I have mentioned above - the goal now is to just bring down the cost, which history shows is inevitable. For example, the LED was first designed back in the 1920s or 1930s but it wasn't until the 2000s that a manufacturing process for building these LEDs became affordable. My whole point here is that the additive manufacturing industry just might be one hell of an industry to start investing in because it seems we're only a decade or less away from most things being 3D printed (no basis for that number, just mere speculation).
If I had known Amazon, Apple, etc. would be as big as it is today, I would have definitely invested in them 10 years ago, which leaves us asking ourselves "Which company should I invest in now and just sit on for 10 years to pull a massive profit?" and to that I say do some research into companies in the additive manufacturing industry and it just might be a very wise investment.