The ferns are an ancient lineage of plants, dating back to at least the Devonian. They include three living groups -- Marattiales, Ophioglossales, and leptosporangiate ferns -- as well as a couple of extinct groups. An additional group, the Psilotales, is tentatively included in the ferns, though the group is so vastly different from living ferns that no one is really certain of its relationships.
For more about ferns, visit the American Fern Society. The AFS maintains a nice list of fern-related links.
For more information on fern relationships and fern biology, visit the Filicopsida page on the Tree of Life.