A Web Parts page contains Web Parts in Web Part zones.
Site pages are pages that are created, edited, and customized by end users. They are primarily used for the content in a site. Site pages come in two types—a standard page and a Web Parts page. A standard page contains text, images, Web Parts, and other elements. A Web Parts page contains Web Parts in Web Part zones. They have a predefined layout that uses Web Part zones. Both types of site pages are edited using a Web browser or Microsoft SharePoint Designer.