Mercury rotates in such a way that one face is always pointed towards the sun. This, along with it's extreme proximity to the star, means that one side of the planet can get extremely hot, up to 700 K. By Earth standards, that would kill every living thing on the surface of the planet. This extreme temperature, however, is nothing compared to the internal temperature of planets like Jupiter. The face pointing away from the sun gets extremely cold. Even though the planet is so close to the star, there is no atmosphere to circulate the heat, and because this dark side never gets any sun, it can get down to as low as 90K.
Half of Mercury is forever bathed in shadow.
Mercury is a heavily cratered planet with no natural satellites. It's surface is very flat and closely resembles that of the moon. Mercury is one of only 4 terrestrial planets in our solar system, all the others are gaseous. The planet is 70% metallic and 30% silicate. It is smaller than the largest moons in the solar system, but it is more massive. Unlike our moon, this planet has an iron core which is enough to provide it with a magnetic field, though weak one. The magnetic field around Mercury is only 1% the strength of Earth's.
Surface of Mercury, taken by Messenger probe.
So far, little is known about the planet. We can only observe a small amount of it using terrestrial telescopes, and only one space probe has been able to map it so far. Another probe has been sent and will stay in orbit around Mercury to map it's surface, however, so we will one day know much more.