Superorganisms are theoretical entities consisting of a massive conglomeration of lesser organisms. Under this definition, of course, such entities are hardly theoretical- plants and animals are conglomerations of massive numbers of individual cells, and so could be regarded as superorganisms. However, this term is more often used in a more exotic context.
Superorganism is a term more often applied to large structures such as cities. Like biological life, cities take in nourishment (via the importation of goods, services, and people), circulate that nourishment to areas in need of it (via the use of vehicles and streets, similar to veins and arteries transferring nutrients), excrete waste (via... waste disposal), and have a central pump through which the circulation is driven (as all cities have "hearts" through which traffic constantly flows). Cities can also be regarded as parts of larger superorganisms (e.g. conglomerations of cities form countries, countries conglomerate to form the planet, planets form solar systems, etc., etc.).
Ultimately, the universe is the largest existing superorganism.