The second planet from the sun, Venus is named for the Roman goddess of love and beauty (her Greek name is Aphrodite). Venus has the distinction of being the brightest object in the sky aside from the sun and the Moon. Its brightness is not only the result of the planet's orbit's (relatively) close proximity to that of Earth (about 0.277 AU) but the reflection of sunlight off its cloud-covered surface. Venus' atmosphere is covered by several layers of thick clouds (several kilometres thick, in fact) composed of sulfuric acid. These clouds produce an advanced form of the greenhouse effect (trapping heat that enters the atmosphere from the sun, raising the global temperature), creating a mean surface temperature of 726 Kelvin. Venus' surface temperature is actually hotter than Mercury's, despite Mercury being much close to the sun. (Venus' mean distance from the sun is 0.723 AU. Mercury's mean distance from the sun is 0.387 AU.) Beneath the clouds the atmosphere is made up of 96% carbon dioxide, 3% nitrogen, and 0.003% water vapour.
The atmospheric pressure on the surface is roughly 90 atmospheres (around the same pressure that would be found 1 kilometre deep in an ocean on Earth). Higher cloud layers can reach speeds of 350 kilometres per hour but on the surface winds are very slow. The surface itself is mostly plains with a few large depressions and two continent-sized mountainous areas named Aphrodite Terra (which is near Venus' equator) and Ishtar Terra (which is in the northern hemisphere). Much of the surface is covered with huge lava flows and it appears that Venus is still very voncanically active in a few areas.
One feature of the surface that has greatly puzzled scientists is the dramatic lack of craters on the surface when compared to other planets such as Mercery or Mars or satellites like the Moon. There are several possible explanations for this: First, Venus is believed to be a relatively young planet, much like Earth (the two planets actually have quite a bit in common -- more on that later), and therefore hasn't been exposed to as many meteorites as some other planets. The dense and ultra-hot atmosphere also probably burns up many meteors before they can strike the surface. Anything large enough not be completely burnt up in the atmosphere likely breaks up before it impacts with the surface, which would explain why the few craters Venus has are located bunched together. Another theory about why the surface of Venus is largely unscarred is that around 800 million years ago volcanic activity engulfed the entire surface of the planet in magma, thus melting away any discernable impact craters. If this theory is true, it is possible that volcanic activity may once again engulf the surface someday. There is also some evidence that the surface of Venus spreads and folds on itself (but there is no evidence of plate tectonics). Not much is known about this but it may have to do with the drastically higher surface temperature.
Venus takes approximately 224 days, 16 hours, 49 minutes, and 26 seconds to revolve around the sun. A day on Venus (the time it takes for the planet complete one full rotation) lasts 243 days, 26 minutes, and 56 seconds. Yes, Venus' day is longer than its year by almost ten Earth days. Not only this, Venus also rotates retrograde Earth and most other planets in its solar system (possible reasons for this are already covered in bigmouth_strikes' write-up). The slow rotation of Venus is thought to be the reason that planet has no magnetic field.
Venus shares a number of similarities with Earth and, prior to the discovery of what the environment on the surface was like, was thought by many to possibly be home to lifeforms. Venus' diametre (12104 kilometres) is just under 95% of Earth's (12756 kilometres). The mass of the two planets is also close (though not as much): Venus with a mass of 4.87 x 1024 kilograms and Earth with a mass of 5.98 x 1024 (Venus' mass is roughly 81% that of Earth's). The mean density of Venus is 5250 kilograms per cubic metre (roughly 95% that of Earth's).
Unlike Earth, Venus has no natural satellites. Between 1672 and 1887, however, there was some speculation amongst astronomers about whether or not Venus did, in fact, have no satellites. Over the course of those years several astronomers claimed to have seen a small object moving with Venus. Other astronomers looking for the reported moon, eventually named Neith by astronomer M. Hozeau in 1884 failed to have much luck in finding the mysterious object (Neith was the name of an ancient Egyptian goddess said to wear a veil never lifted by a mortal). In 1887, the Belgian Academy of Sciences published a lengthy report investigating every sighting of Neith up until then. It was concluded that the occasionally seen satellite of Venus was, in fact, a number of different stars that appeared close to the planet at the time the supposed moon was observed.
The first visit to Venus was made in 1962 by the Mariner 2 spacecraft. Since then over 20 craft have been sent to the planet (none manned, as the planet's environment is very unfriendly to not only life but various metals). The Soviet spacecraft Venera 7 was the first craft from Earth to land on the surface of not only Venus but any planet other than Earth and did so in 1970.
Sources:
http://www.seds.org/nineplanets/nineplanets/venus.html
http://www.seds.org/nineplanets/nineplanets/data.html
http://www.seds.org/nineplanets/nineplanets/hypo.html
http://www.seds.org/nineplanets/nineplanets/spacecraft.htm
http://pds.jpl.nasa.gov/planets/welcome/venus.htm
http://pds.jpl.nasa.gov/planets/welcome/earth.htm
http://www.wikipedia.com/wiki/Venus_the_planet