A phylum - also known as a division when referring to plants - is a scientfic way of grouping together related organisms. All the members of a phylum have a common ancestor and anatomical similarities. For instance, all the arthropods have external skeletons. Phlya are large groups and are further subdivided into classes, orders, families and so on.
In biological classification, rank is the level in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, and class. Each rank subsumes under it a number of less general categories. The rank of species, and specification of the genus to which the species belongs is basic, which means that it may not be necessary to specify ranks other than these.