Anthozoans are divided into three main subclasses. The Zoantharia includes the "true" corals and most sea anemones, and it is by far the best represented in the fossil record. On the cladogram pictured above, the Zoantharia is shown subdivided into constituent clades: the Scleractinia (living true corals); Zoanthiniaria, Corallimorpharia, and Actinaria (collectively known as sea anemones); and Rugosa, Heliolitida, and Tabulata (extinct groups of corals).
A second subclass, the Octocorallia, includes the sea pens, soft "corals", and sea fans. This clade has a less well-documented fossil record than the corals, but many of its members form mineralized, microscopic spicules.
The third subclass, the Ceriantipatharia, is very rare in the fossil record; it includes certain burrowing "sea anemones" (Ceriantharia) and precious black "corals" (Antipatharia). Some zoologists prefer to regard these two as separate groups and do not believe that they form a monophyletic group.
See the Anthozoa page from the Tree of Life website.