A group web calendar is like a hallway calendar--it's a very visible calendar that everyone in a group can edit. It's very good for highlighting important dates. However, you can't send invitations through group calendars.
Because the web calendar uses iCal server, all users who have set up their server account in iCal will have synced calendars. Event changes made in the web calendar are reflected in iCal, and event changes in iCal are reflected in the web calendar.
If you sync group web calendars, the local iCal version of the calendars are read-only. Any changes you make to the group web calendars are synced with iCal but you can't edit the calendars in iCal.
The web calendar simplifies creating events and sending invitations. Web calendars don't allow dependencies, so it's not good for project management where milestones often depend on each other. It also doesn't allow delegates, so you should use iCal if you want to use delegates.