The cells of a CCD sensor operate as exposition meters accumulating charge and are read out in certain intervals. The »fast« CCD sensors exist in three types: FT ((Full) Frame Transfer), ILT (Interline Transfer) and FIT (Frame Interline Transfer). For still photography the full frame principle is sufficient enough: The charges remain in the active area until read-out shaded by a central mechanical shutter.
In FTs the complete frame is moved through the photo active cells in a separate light shielded area and then it is processed. Due to the different structures often it is possible to identify both regions by one's naked eyes. ILTs show alternating photosensitive and read-out lines. The charge of each photoelectric cell is directly pushed to the according light shielded neighboring cell and this line is then read-out. FITs are a combination of both designs.
FTs reach a fill factor of almost 100%, but suffer of a second exposure during the read-out phase (so-called smear). ILTs and FITs show a reduced fill factor, but are less sensitive in read-out phase.