Each cell can be locked or unlocked. And a worksheet can be protected or
unprotected.
So if you want to allow the user to make changes to certain cells, you can
unlock those cells and then protect the sheet.
If you haven't changed the default style, every cell is locked by default. (And
I bet you haven't!)
Select your range to unlock
Format|cells|Protection tab
uncheck locked
Now protect the worksheet.
tools|Protection|protect sheet
And in versions of excel before xl2002, you may find some stuff (formatting
cells, for instance) that you want to do is unavailable on a protected
worksheet.
That said, worksheet protection is very weak. There's code posted here every
day/week that would unprotect the worksheet.