When a pesticide is used, sometimes not all of the targeted pests are killed. Pests that are not killed may possess a genetic change or mutation that allows their bodies to deal with and breakdown the pesticide without dying. Then, these pests will mate and pass on the ability to deal with that particular pesticide to their children. The cycle continues that way and the pesticide is no longer harmful to that pest population.
Because resistance builds up this way, it's good to "rotate chemistries", meaning using insecticides with different chemical make-up to target a group. Using a differently named pesticide that has the same chemical make-up (like two different organophosphates) will not work because the pesticides work the same way.
Also, it's important to always follow the label directions and use the recommended amount of pesticide. When you use less than the recommended rate you run the risk of not killing the target pest and therefore help build resistance.
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Find more information about pests in eXtensions Pest Management In and Around Structures Resource Area.
