A porphyrin is a heterocyclic aromatic ring made from 4 pyrrole subunits joined on opposite sides through 4 methine links. The ring, therefore, is completely aromatic, unlike the related corrins or chlorins. Porphyrins combine readily with metals, coordinating with them in the central "hole". Iron, zinc, copper, nickel, and cobalt containing porphyrins are known. Iron containing porphyrins are called hemes, and heme containing proteins, or hemoproteins, are found extensively in biochemistry.
If one of the four pyrrole subunits is reduced, a chlorin is produced, the ring structure found in chlorophyll. If two of the four pyrrole subunits are reduced, then you get a bacteriochlorin, found in some photosynthetic bacteria.
See also: porphyria