The term phylum is one of seven major categories that are used to classify organisms. In order of broad to specific, these seven categories are: Kingdom, Phylum, Class, Order, Family, Genus, and Species. Carolus Linnaeus developed this system of classification in the 18th century.
Any organism that has been identified and scientifically described is put into each of the seven categories above. Studying an organism's classification can tell us what other organisms it is similar to.
Marine organisms are divided into many different phyla. All animals with a notochord (vertebrates) are in their own phyla, the Chordata. The rest of the animals are divided up into a very diverse array of invertebrate phyla. Some of the more well-known invertebrate phyla are include the: echinodermata, arthropoda, annelida, mollusca and Cnidaria.