You can do this a number of simple ways...

Each sheet can have your data in a standard format (say for regions -
each sheet would have that regions numbers in it) and have a summary
sheet that references all of the other sheets - if they are formated
the same, it is as simple as entering the = sign in the summary cell,
and then selecting the cell in each sheet you want to add into that
cell one at a time wile pressing the + key to add them up. You only
need to build this formula once, then you can copy and paste to the
other summary areas of your summary sheet.

hth
chris