Trojan asteroids are objects that share a planet's orbit around the Sun but reside in what are basically gravitational dead zones. We've discovered Earth's first Trojan asteroid...but how did it get there, and why have we only just found it?
In any system where a smaller object orbits a larger one - say the Moon going around the Earth, or the Earth going around the Sun - there are five places where the gravitational influence of the two objects cancel each other out. These are known as the Lagrangian points. In the Sun-Earth system, one of these points are found directly on the other side of the Sun from Earth, two are found in the immediate vicinity of Earth along the Sun's extended radius, and two more are located 60 degrees away from Earth in either direction on its orbit. You can see a diagram of these points below.
An object located at any of these points can remain at a constantly fixed position relative to both the Sun and the Earth - in other words, it shares both the Earth's orbit and its orbital speed. As you can see from the diagram, we number the different Lagrangian points: L1, L2, L3, L4, and L5. L1 and L2, being the closest to Earth, are the ones we most often take advantage of, placing solar observatories at L1 and space telescopes at L2.
L3 is the one on the other side of the Sun, and it's where science fiction tends to put hidden "Counter-Earths", objects that share our orbit but remain forever hidden. (You, uh, may have heard of one such example of this.) However, while the notion of an object hidden at L3 is enticing, the physics of it simply don't work - the gravitational influences of other planets, particularly Venus, make L3 far too unstable to support even a small object for very long.