The X-37B isn't a spaceship, and it isn't a plane. It's somewhere in between. Looking like a mini space shuttle, the spaceplane is 29 feet long, has a wingspan of 15 feet, and is built by Boeing. Notably, the X-37B is unmanned by pilots. It's also solar powered (like NASA's space shuttle) and most importantly, it's the only model of reusable spaceplanes in the world.
Although it seems strange that something as sturdy as a rocket is single use, like a coffee cup, this has been one of the major setbacks for space travel research. Although reusable spacecraft has been imagined since the beginning of the Space age, it's actually not that easy to get a rockets back onto Earth in one piece.
Rockets are built to burn a lot of fuel and discard parts as they enter new levels of the atmosphere. What's more, getting back to Earth from space is never a smooth ride. Rockets usually crash back onto Earth, rather than land, or burn up in the atmosphere, or remain in orbit as space junk. This is the reason why space travel is so expensive, which in turn hinders potential progress in space exploration.
That's where spaceplanes come into the picture, and the X-37B is the only reusable space plane model that exists today. The robotic machine takes off like a rocket with the help of a rocket booster, but lands like a plane under it's own power.
The Air Force is extremely secretive about the purpose of the X-37B: it has already been on four missions that lasted over two years each. In a statement about the spaceplane's landing, Secretary of the Air Force Barbara Barrett said "The X-37B continues to demonstrate the importance of a reusable spaceplane. Each successive mission advances our nation's space capabilities."
The purpose of the spaceplane's 780 day mission isn't elaborated on, but there have been hints that in the most previous mission, the spaceplane traveled to higher-inclination orbits than before, suggesting that new experiments or technology tests are underway.
There's no doubt that as the technology advances, and spaceplanes become more accessible, the public will learn more about their uses. Who knows, perhaps we'll all be able to catch spaceplanes to the moon one day.