Mercury is the planet closest to the sun.
Therefor the side of the planet that is in the light of the sun has a temperature of 200 degrees Celcius (392 degrees Fahrenheit). On the other side of the planet the temperature is at the same time -400 degrees Celcius (-688 degrees Fahrenheit).

The sun shines for only three months a year. That's because Mercury rotates around it's axis very slowly (once every 58.6 days). Because of these extreme temperatures, and because Mercury doesn't have an  atmosphere, no life can exist on the planet. 

Mercury has a large iron core which is between 1,800 and 1,900 km thick.
The outer shell is about 500 km thick.
The planet has no satellites.

Mercury rotates around the sun faster then any other planet of the solar system. It takes mercury 88 days to complete an entire rotation around the sun. This also means that mercury rotates around it's axis 3 times for every 2 times it rotates the sun. Scietists used to believe mercury rotates around it's axis just as fast as it rotates around the sun, and that therefor just one side of the planet was turned to the moon (just like the moon is to earth).

So far, mercury has been visited by only one spacecraft, the mariner 10.
This spacecraft flew by mercury 3 times in 1974 and 1975, but only mapped 45% of the planets surface.

The diameter of the planet is 4,878 km (3032 miles) 
The distance from the planet to the sun is 57.91 million km (35.99 million miles) and the planet has a mass of 3.30e23 kg

Pictures of Mercury (click the image to see a larger view):