Mercury is both a chemical element and a planet.
Mercury the element is also known as quicksilver because at room temperature it's a silver liquid. Its symbol on the periodic table, Hg, is short for hydrargyrum, which means "runny silver" in Greek. Common uses of mercury include thermometers, fluorescent light bulbs, and dental fillings. Mercury is also a well-known industrial pollutant, chiefly from coal combustion, and is toxic to both humans and wildlife. Mercury poisoning is especially an issue in seafood, where the higher up a food chain a fish is, the more mercury it has stored in its flesh.
The planet Mercury is so close to the Sun that it takes Einstein's general theory of relativity to explain how its orbit moves under the Sun's immense gravitational influence. A year on Mercury is 88 Earth days long. A sidereal day on Mercury (one full rotation on its axis) is about 59 Earth days long, but a solar day (from noon to noon) is nearly two Mercury years long. Mercury rotates three times for every two orbits around the Sun, and in some places the Sun appears to rise halfway, go back the other way, then rise again. Mercury is the smallest planet but also the second densest, after Earth. It has no moons. Currently, NASA's MESSENGER spacecraft is in orbit around the planet for a detailed study.
(Liquid mercury picture by Bionerd, used under CC-A-3.0 license. Planet Mercury picture by NASA.)