Mutualism is a symbiotic relationship in which both species benefit from the interaction. For example, bees and flowers have a mutualistic relationship. Bees collect nectar from flowers, which the bees use as a food source. In the process of collecting nectar, bees also transfer pollen from one flower to another, which helps the flowers to reproduce.
Commensalism is a symbiotic relationship in which one species benefits from the interaction, while the other species is neither harmed nor helped. For example, birds that build their nests in trees benefit from the protection that the trees provide, while the trees are not affected by the birds.
Parasitism is a symbiotic relationship in which one species (the parasite) benefits from the interaction, while the other species (the host) is harmed. For example, fleas and ticks are parasites that feed on the blood of animals, which can cause health problems for the animals.