(More info, and a better description...)

With the possible exception of a single HST servicing mission, the
Space Shuttle is now entirely dedicated to flights servicing the
International Space Station (ISS). ISS orbits in a specific "orbital
plane", which can be roughly visualized as a sheet of paper running
through the center of the Earth, and tipped at an angle of 51 degrees
from the equator, so that the paper forms a circle touching all lines
of latitude between 51N and 51S. This inclined orbit was chosen to
allow launches to ISS from both American and Russian launch sites
(although the Russian launch site is actually in Kazakhstan).

When the shuttle flies to ISS, the exact position of ISS in its orbit
is not terribly important, because it is relatively easy to shift an
orbiting shuttle forward or backward in the orbital plane. What
matters is the position of the orbital plane itself, because it is
quite difficult to change planes in space. Therefore, the Shuttle is
launched at the exact time when the Earth's rotation brings Cape
Canaveral through the imaginary sheet of paper that defines the ISS
orbital plane. This creates a very short launch window.

The shuttle's launch trajectory follows the path defined by that
imaginary sheet of paper, tipped 51 degrees to the equator. This path
takes the shuttle northeast along the East Coast of the United States.
(Technically, there is a second launch opportunity each day to the
southeast, but that trajectory cannot be used because the launch
trajectory would take the shuttle over populated regions of the
Caribbean during the launch process.)

When the shuttle launches at night, the launch is actually visible
throughout the Atlantic coastline. Main engine cutoff, eight minutes
after launch, happens off the New England shoreline
(end)