A taxon (plural taxa), or taxonomic unit, is a name that means an organism or group of organisms.
One has to distinct between taxa/taxonomy and classification/systematics. Taxonomy refers to biological names and the rules of naming. Classification refers to rank ordering of taxa according to presumptive evolutionary (phylogenetic) relationships.
A broad scheme of ranks in hierarchical order:
A simple phrase to remember the order is "Dignified Kings Play Chess On Fine Green Silk".