All four of these words describe the way in which an organism acquires nutrients. A key word used in describing these relationships is "food chain". This is not a particularly accurate description, however.
The origin of the energy that goes into making the food is mostly from light, though there are some exceptions such as extremophiles. Herbivores only prey on plants. Carnivores only prey on non-plant organisms, that is, heterotrophs. Omnivores can prey on plants and other organisms. Saprobes, better known as detrivores, decompose living/dead organisms. Some saprobes eat plants, some eat herbivores, carnivores, or omnivores. Saprobes may also eat already eaten organic matter to further break it down, deriving energy.