The SPSite class represents a collection of sites contained on a virtual server, which includes the top-level (root) site and all its subsites. Each SPSite object can be found within the Sites property of an SPWebApplication class.
The SPWeb class represents an instance of a single website. It is often confusing to developers because the root web of a site collection (which is created when a site collection is created) is an instance of the SPWeb class.