Answers Your Questions
From: Mike Powney, of Epsom,
The way orbital mechanics work, you raise your orbit by executing a burn to speed you up. The reboost burns for the ISS can be executed by a visiting shuttle or progress vehicle and even by the engines on the ISS' Service Module. A typical reboost may raise your orbit 6 miles or so. The reboost strategy employed by ISS depends on ISS attitude changes, visiting vehicles, etc. Just like an airplane is more (or less) aerodynamic depending on the side that is facing "forward", ISS is more or less aerodynamic depending on which attitude it is in. Certain attitudes create more atmospheric drag than others.
Also, if a visiting vehicle,
such as the Space Shuttle, is planning to rendezvous with ISS in the
near future, ISS will be allowed to decay to a lower orbit so the Shuttle
does not have to expend as much energy to rendezvous at a higher orbit.
This allows for the Shuttle to haul up more payload mass. Finally, you
are correct, this does mean that without these periodic reboosts, the
ISS orbit would decay and eventually de-orbit.