A site collection is a container for one or more sites that is usually created by an administrator at the request of a group of users. Every site collection is based on a template that specifies the initial configuration and content of the site collection. Depending on the version of SharePoint you are using, the stock enterprise types are listed to the right.
Every site collection contains at least one site called the top level site. This site is sometimes refereed to as the root web because it is at the root of the potential hierarchy of other sites that exist as children of the top level site. The top level site is special because it contains configuration options that govern the behavior of the entire site collection.
The other sites that exist under the top level site are called subsites. Both top-level and subsites are usually just called sites.