Centrifugal juice extractors are the most popular type of juicers; they can juice most any fruit or vegetable you can throw at it. These machines grate your food, and then spin the juice out at high speeds.
If you want to juice leafy greens, herbs, and wheatgrass… then you’ll need a Masticating juicer. If you want to simply save yourself the trouble of squeezing an orange or lemon… then a Citrus juicer will do. But most people will be happiest with one of the Centrifugal juicers above. They are powerful, easy to use, relatively easy to clean, and have extra wide mouths to handle whole fruits. The more expensive ones will be somewhat quieter and have more power (yielding slightly more juice), but these all operate in basically the same manner.