http://spaceflight.nasa.gov/shuttle/reference/basics/landing.html
Landing
When it is time to return to Earth, the shuttle is rotated
tailfirst into the direction of travel to prepare for another firing of
the orbital maneuvering system engines, a firing called the deorbit burn.
This engine firing, usually about three minutes long, slows the shuttle
by only a couple of hundred miles per hour, but it is enough that it
begins to descend toward the atmosphere.
The engine firing takes place usually half a world away from the intended
landing site: for example, the firing may take place above the Indian
Ocean to put the shuttle on course toward a landing at the Kennedy Space
Center. The three-minute firing is the only active brake the shuttle will
use as it heads toward a landing.
The rest of its descent toward Florida and trip halfway around the world
is devoted to slowing down using only the drag produced by the
atmosphere.
After the firing takes place, it is about another 25 minutes before the
shuttle will descend to a point that it first encounters the effects of
the atmosphere, usually at an altitude of about 129 kilometers (80 miles)
and a range of more than 8,047 kilometers (5,000 miles) from the landing
site. Before the shuttle encounters the atmosphere, leftover fuel is
burned from the forward reaction control system steering jets as a safety
precaution. Before it reaches the upper atmosphere, the shuttle is
oriented with the nose angled up about 40 degrees from horizontal and its
wings level, an orientation that keeps the black thermal tiles on the
underside facing the majority of the heat generated by its encounter,
heat that can range as high as 1,648.9 degrees Celsius (3,000 degrees
Fahrenheit) on the leading edges of the wings and nose.
The aft steering jets are used to control the shuttle's orientation as it
descends into the atmosphere. As it descends, however, it begins a
transition from spacecraft to aircraft, and its aerosurfaces -- the wing
flaps and rudder -- gradually become active as air pressure builds. As
those surfaces become usable, the steering jets turn off automatically.
