Linux is widely known as a free operating system you can run on your PC. It runs on a lot of other things too. In fact, if you have an Android phone, you are running Linux on it. My TV is running Linux, so is my router. The list goes on.
Q: Linux is free, Windows cost $199. They do basically the same thing, how is this possible?
A: It is Open Sourced.
This confused me for a long time as well. The best way I can explain it is, someone with talent gets an idea but doesn't have a capital to develop it or market it. So, they focus on making it a viable product and give it away. The more useful it is, the more popular it becomes. Other people help and eventually enough people are using it that it becomes indispensable. By this time, the people who put in the sweat equity have more bills to pay (they've progressed from early adulthood to twenty or thirty-something) and they're able to charge a fee for consulting, support or customization services.
Basically, the originators of an Open Source project use sweat equity to create a career for themselves. It's also something they love to do, so basically they never work a day in their lives. This is brilliant and bold.
I've personally never had the gumption to join an Open Source project. First off, you need to convince the team you're an asset or they won't let you join. Secondly, they're pretty serious about what they are doing so you need to be committed. Thirdly, you need to produce a quality product and they check it throughly.
Software projects are the most common Open Source projects. The most established place it was available is SourceForge. Google Code was a popular place as well, but it closed down when Github was established. Today, Github is the definitive place for Open Source projects to be kept.
Although software projects are most common, Open Source hardware projects exist as well. Some of them are huge, like the Hyperloop. PowerPC is a chipset used in Cisco routers and a bunch of other computer equipment, IBM donated it. Facebook's data centers are Open Source. The servers, the racks, the buildings themselves.