In Dreamweaver, the word site is used as shorthand to refer to any of the following things:
A website: a set of pages on a server, to be viewed by a visitor to the site using a web browser.
A remote site: the files on the server that make up a website, from your (the author's) point of view rather than a visitor's point of view.
A local site: the files on your local disk that correspond to the files in the remote site. In the most common Dreamweaver workflow, you edit the files on your local disk, then upload them to the remote site.
A Dreamweaver site definition: a set of defining characteristics for a local site, plus information on how the local site corresponds to a remote site.