Yogurt is a prebiotic, primarily. If you buy most commercial yogurts, there are no live organisms in them, as they are killed during pasteurization. If you buy very high quality yogurt, usually organic ones in the health food stores, they will add live organisms back in after pasteurization, but even then, there are very few of them compared to the billions you get in a good quality refrigerated probiotic found in the refrigerated section of your best local health food store (shelf stable probiotic supplements probably have more dead than live organisms in them by the time you buy them, as these are living things that must either have food to eat and a habitable environment, or hibernate, which they do when kept under refrigeration). But yogurt is still a good prebiotic, so again, you're eating one. Unfortunately, that one happens to be a dairy product, and that raises the many problems with consuming dairy, but if you are going to force-feed something not natural for an adult mammal to eat into your body, fermenting or culturing it makes it a lot less of a problem. So that's one thing. But the two categories, while connected, are also separate. A lot of supplements contain FOS, a sugar that is a good food for probiotics, or in other words, they have a prebiotic in them. But there are only a few beneficial organisms we know about that are safe to consume, and so you'll notice there aren't a ton of them available for consumption but there are a ton of them in your body. That means that eating good prebiotics will provide food for all those other organisms as well, so using both is better than using one or the other. And you might actually be eating prebiotics without knowing it -- if you like fermented and cultured foods, such as pickles and kim chi and sauerkraut and tempeh and miso and all the multitudes of these kinds of foods people eat then you are in fact eating prebiotics as long as you're not cooking them to death.