While some businesses are owned by individuals, many involve multiple owners. Multiple owners may share in the day-to-day responsibilities of running the business, or they may be silent partners who share ownership without actively participating in the business. The way they're described also depends upon the business's legal structure.
The term "partner" designates a co-owner, usually within a business that's organized as a legal partnership. Partners each own some portion of the business and are legally responsible for the decisions of all other partners. Partners may be actively involved in operating the business or may be silent partners who participate financially with no day-to-day business responsibilities.
A principal also refers to a co-owner, irrespective of the type of legal business structure. Principals are often also co-founders, and the title implies both ownership and primacy, as in one who is among the first to join a company or venture. Like a partner, a principal may be involved or uninvolved in actually operating the business.
The title "co-founder" does designate a co-owner of a business, but it also denotes one who helped to found or give birth to the business. Co-founders are distinguished from partners or principals in that they have participated in the inception of the business, irrespective of their continued participation. Unlike partners or principals, most co-founders continue to participate in actively operating and growing their businesses.
Co-proprietor identifies someone who not only co-owns a business but also is actively involved in operating the business. While not exclusively used for small businesses, the term "proprietor" does connote a business that is small enough to be operated principally by the owner or co-owners.