A site is usually a physical location with its own address. It could be a building, a building site or an office.
A zone is normally an area within a site that you want to track access to.
So a building would be a site, but a floor within that building could be a zone. Even a meeting room could be a zone.
You can have as many zones as you need within a site and call them anything you want.
Pricing is based on the number of sites.