In a perfect world, everything we ate would be organically produced. But in reality, sometimes we have to make choices. Not all produce items deal with pesticides and chemicals in the same way. Some soak the toxins in, and others make it to harvest relatively unscathed.
Researchers at the Environmental Working Group developed a list of "The Dirty Dozen" fruits and vegetables. They recommend that you always buy these items organic when possible, because their conventionally grown counterparts are laden with pesticides.
The "Clean Fifteen" is a list of produce items that contain relatively less pesticides when grown conventionally, so you can feel safe purchasing these when organic items aren't available.