Well, let's start at the beginning. The space shuttle has to escape Earth's gravitational attraction by achieving what is known as escape velocity, about 11 meters per second. Once it is in the outer edges of Earth's atmosphere, it assumes an orbital velocity of around 8 meters per second, which keeps it in orbit around the Earth.
When the shuttle is in orbit, and is ready to come back to Earth's surface, it fires retro rockets to slow it's velocity. This will cause the Earth's gravitational attraction to start pulling it towards the Earth. The shuttle is carefully positioned to enter bottom side first, because it has heat tiles along the bottom, designed to reflect the tremendous heat generated upon reentry to the atmosphere. A defect here can be disastrous, as happened to one unlucky shuttle crew.
Once the shuttle has gotten past the reentry phase, the astronauts fly it in like a glider, positioning it to land like an ordinary aircraft. All this is done with air pressure supporting the wings against the Earth's continual gravitational attraction. As it lands, drag chutes pop out to help slow it's velocity.