While everyone else's explanations are pretty adequate at explaining what a co-op is one thing that everyone left out is the crazy maintenance fees, these tend to go up every year. I've got friends who have paid off their units but their monthly maintenance fees are more than what some people pay in rent. Every co-op board is different but they're also a lot harder to sell should you choose to move out. Some co-op boards can also make your life difficult. YMMV.