Sponsors are generally the larger firms that run display ad campaigns here. These companies usually provide "essential services" for traders, and are almost always brokerage firms and software providers. Their ads run around the clock in primary areas, and in the case of something like a forum sponsorship, the exposure is totally exclusive, meaning that no other sponsor can run display ads in a forum that is exclusively sponsored.
Vendors are individuals or small companies that pay a small monthly fee to be able to talk about their offerings on the boards, and have a signature file that gives a brief summary of their company and provides users with their contact info. Vendors are held to a higher standard than regular members, and therefore must use their real names in either their usernames or in their signature files.