The automobile industry is an example of an oligopoly. So is the hospital services industry in most medium to large cities. For example, I said that the automobile industry is an oligopoly because, for example, Toyota's pricing and product decisions have a noticeable affect on the demand for Honda cars. Tobacco growers are not an oligopoly. If one farmer decides not to plant this year, the neighboring farmers won't notice any difference in the market for their tobacco.
Structure is the context in which the actors in the market make their decisions.
Conduct is what decisions the actors make. Performance is what we get as a result in terms of efficiency and equity. Sound applet Monopoly, oligopoly, and perfect competition are different types of market structure, representing different amounts of market concentration. Monopoly is complete concentration. Perfect competition is no concentration.