Each cell can be locked or unlocked.
(Format|Cells|Protection tab in xl2003 menus)
But that "lockedness" doesn't really do much until you protect the sheet.
(tools|protection|protect sheet in xl2003 menus)
So you can lock/unlock the cells you want, then protect the sheet. (Lock the
cells with formulas and headers/descriptions/... and unlock the cells that the
user should be able to change.)
Be aware that worksheet protection is easily broken.
And you may want to test it before you release the workbook to others. There
are lots of things that can't be done on a protected worksheet.