One of the primary capabilities of SharePoint is the ability to open, edit, and
then save documents, in this case Excel spreadsheets, directly to document
libraries. So yes a user can open an Excel spreadsheet and save it directly
back to SharePoint. this can be done with several versions of Excel, but you
will have the best level of integration if you use Excel 2007. The one thing you
won't be able to do is have two users editing the same spreadsheet at the same
time, but that's a limitation of Excel not SharePoint.