Venus is covered by a carbon dioxide atmosphere with toxic sulfuric acid clouds. At the surface, the pressure is about 92 atmospheres, similar to being under a kilometer of water on the Earth. Its surface is the temperature of an oven, averaging 863 °F (462 °C). The equator is even hotter, at 932 °F (500 °C), sufficient to melt lead. When the Soviets sent specially-armored probes to the surface of Venus, they only lasted between 20 and 40 minutes before succumbing to the extreme pressure and heat.
Despite these harsh conditions, Venus is appealing to colonize. With similar mass to the Earth, it would offer similar surface area and familiar gravity levels, unlike small, low-g Mars. It is also closer to Earth than Mars is, and its proximity to the Sun would provide greater solar power per square foot of solar panels.
Unfortunately, colonizing the surface of Venus would be very difficult. You'd either need to create cybernetic humans that can operate in such harsh conditions, or somehow solidify a large part of the atmosphere to lower its density. This would require planetary engineering of Venus, a truly energy-hungry and logistically challenging process. The process of transforming a hostile planet to a more Earth-like environment has been called terraforming.
More realistic in the coming century or two is colonizing Venus's upper atmosphere. At an altitude of about 50 kilometers, the air pressure is similar to that on Earth's surface. Because breathable air serves as a buoyant gas in the atmosphere of Venus, floating domes filled with air would be similar to balloons, allowing floating platforms. The winds here are intense enough to blow a floating platform around Venus about every 100 hours, leading to a 100-hour day, highly preferable to depending on Venus's natural rotation, which makes one full revolution only every 243 days!
Obtaining breathable air for humans could be done by processing Venus' atmosphere. The carbon dioxide, making up 96.5%, could be processed to provide oxygen. Nitrogen, which makes up 78% of our own atmosphere, makes up 3.5% of the Venusian atmosphere, could also be extracted and purified in large quantities.
