The site owner is the staff or faculty member who has overall responsibility for the site; it is generally someone who doesn't change positions frequently, like a departmental Administrative Support Coordinator (ASC) or Administrative Support Assistant (ASA), rather than a department chair. Students are not permitted to own a site on the Central Web server.
Site developers are the people who actually make changes to the site. This may be the person who designed your site, or the person who updates content, uploads files, etc - which may include students owners pay to make updates - or anybody else who logs into your site to make changes. Developers are sometimes also site owners.