These slender, wedge shaped insects get their name because they have a tendency to jump when approached. They are quite common but don’t usually occur in large numbers and are rarely a big problem in vegetable gardens. They most often attack beets, potatoes and beans (and occasionally other crops) and can spread virus diseases such as Curly Top. They feed by sucking juices out of the plant and commonly cause necrotic patches where they inject toxins into the plant.
Generally you don’t need to do anything about leafhoppers, natural predators will keep them under control. If they do become a problem you can try washing the nymphs off of the plants with a strong jet of water.
Image: John A. Weidhass, Virginia Polytechnic Institute and State University, Bugwood.org