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. Oligopoly is in between. Sound applet Structure determines whether conduct matters. Conduct doesn't matter in a perfectly competitive market, because any firm that doesn't minimize cost and keep its price low is out of business. Conduct matters considerably in monopolized markets.