Joint ventures in online terms are relationships where product creators and marketers come together to promote product to the interests of their customers, of the marketers, and of the innovators. In online joint ventures, there is no sharing of ownership in the products. The marketers are independent contractors.
It is very important to be clear about the online relationship as in the offline world a joint venture is a formal legal relationship that shares ownership, expenses and costs and profits based on the agreement. Generally, in the absence of a written agreement, the split is 50/50. See an attorney for advice about forming a joint venture as this definition is not a legal one but a marketing coaching description. Offline it would normally be considered a strategic alliance.
An online joint venture is basically an affiliate relationship, often called a super affiliate. The main difference is that a JV partner (no shared ownership) typically gets more benefits and the JV partners intend the relationship to run for the long term on multiple products and/or services. Additionally, JV partners often bring in affiliates in exchange for a second tier commission.
More on this concept as applied to offline small businesses: