Recognising different associations of organisms
Corals and the green algae in their cells benefit each other. We say they have a symbiotic relationship (they both get something out of it)
An orchid attatched to the branch of a tree does the tree no harm, but the orchid benefits by getting more light. This is what we call commensalism meaning one benefits while the other is not harmed.
Parasitism is a close association between two organisms of different species in which it is beneficial to one (the parasite) and harmful to the other (the host). Examples of this relationship are tapeworms and the malaria parasite.