Shuttle Processing Explanation (or,  what are all these acronyms
anyway?): 

 The Shuttle consists of an Orbiter (OV), an expendable External Tank
(ET), and a reusable pair of Redesigned Solid Rocket Motors (RSRM).  It
is launched from Kennedy Space Center (KSC). The OV is prepared for
flight in the Orbiter Processing Facility (OPF) which consists of three
bays (one of which is actually a separate building) after which it is
towed to the Vehicle Assembly Building (VAB) and `mated to the stack' or
joined to the ET and RSRM. First, the segments of the RSRM are stacked
up on a Mobile Launch Platform (ML) and then the ET is connected to it.
After the OV is mated, a Crawler-Transporter is moved underneath the ML
and carries the ML/RSRM/ET/OV stack to one of the two pads (A or B) at
launch complex 39 (LC39) where it is eventually launched on a Space
Transportation System (STS) mission. Occasionally an OV is returned to
the Rockwell International plant in Palmdale, California for refit - an
Orbiter Maintenance Down Period or OMDP.