Colonial organisms are actually groups of individual organisms with a close, dependent relationship with other organisms in the colony, often with each member having a very specific specialization that makes them incapable of surviving alone. The individual members can be multicellular organisms or single-celled organisms.
Colonial organisms benefit from their organization by having more protection or being able to catch larger prey than an individual member could. While this description is usually applied to organisms that live attached to one another, insects, such as honey bees and ants, that live in mutually dependent colonies are also sometimes considered colonial organisms.
Some of the most complex examples of colonial organisms are siphonophores, such as the Portuguese man o' war. They are most closely related to jellyfish and sea anemones, which are true individual organisms. These colonial organisms show a very high level of specialization in their individual members, which are known as zooids. The Portuguese man o' war has four different types of zooids, each of which performs a particular function for the colonial organisms that the others cannot. One type of zooid provides propulsion for the colony, while another is able to ingest and digest food items. They live attached to one another, sharing nutrition.