Cruithne is an asteroid, or minor planet, that orbits the Sun with a 1:1 resonance with Earth.
Its unusual in that it is not directly opposite the Sun from Earth (in fact, its orbit occasionally brings it pretty close to us), and its apparent path, when viewed from Earth, is more like a horseshoe, or bean. And even though it is relatively close to us some times, it is harder to see than Pluto.
Some great info on Cruithne on wikipedia http://en.wikipedia.org/wiki/3753_Cruithne and here http://physicsworld.com/cws/article/news/2013/aug/14/earths-second-moon-target-of-proposed-mission and here (with a gorgeous animation that resembles Celtic Knotwork) http://www.wwu.edu/skywise/a101_cruithne.html
Cache is a cammoed pill bottle. Bring your own pen.