Porphyrins (from the Greek for "purple") are a group of organic molecules that share the characteristic of having four pyrolle subunits (made up of C4H4NH groups) connected with methine bridges (=CH-) at the carbon atoms. Heme is the best known of the porphyrins. Porphyrins also are the basic building block of chlorophyll in plants.
In most cases, porphyrins are involved in molecules that hold an atom of a heavy metal, such as the way iron is held within hemoglobin or magnesium in chlorophyll.
Porphyria is a group of diseases where the enzymes involved in the creation of heme are faulty and certain porphyrins are overproduced in the body.