Parasitism is an organism obtains benefits, such as food, shelter, and protection, from another organism with harm. An example of parasitism is a tick with a cat. In order for a tick to survive and grow, it needs blood. For this example, the cat skin is the host and the tick is the parasite. Ticks gets the benefits by sucking blood from a cat. However, the cat as the host get harmed by the tick.
Commensalism is one organism obtains benefits from another different kind of organism with harmless. An example of commensalism is a fish - remora with other larger fishes. A remora follows