Airlines alliances are agreements between many airlines to provide connectivity and marketing. There are many dizzying details about alliances, but all you need to know is that you can exploit them to make your travel as efficient as possible.
There are three alliances: Star Alliance, Oneworld, and SkyTeam. SkyTeam is in a distant third place, in terms of average airline quality and usefulness. The program with which you will likely accumulate the most miles is United Airlines’ MileagePlus, which belongs to Star Alliance.
Star Alliance has 27 full members, such as Asiana Airlines, EgyptAir, and Turkish Airlines. If you fly on any Star Alliance carrier, you can have your miles credited to any program within the alliance. For instance, if you take a Turkish Airlines flight, you can and should credit those miles to United MileagePlus. The reason for this strategy is to minimize breakage, wherein you have small piles of miles in many programs, which is far less useful than having a couple of big stashes.
With your United MileagePlus balance, you can redeem those miles for a flight on any Star Alliance carrier. Thus, having United miles allows one access to at least 26 other airlines!