University of Victoria (Canada)
The U Vic Writer's Guide: Literary Term: Tragedy
The University of Victoria's English department gives a comprehensive definition of tragedy.
New York University
Time Space in Classical Greece: The Poetics
A discussion of the beginnings of tragedy and the six necessary components of tragedy. Includes Aristotle's thoughts on tragedies.
TheatreHistory.com
Theatre history.com: Sophocles and His Tragedies
This site provides a complete biography of Sophocles reprinted from a drama history book. Includes his innovations in ancient theater and his achievements as an artist.
Other
Bob's Byway: Glossary of Poetic Terms
Calling itself "unique," Bob's is easy to use, with cross-links throughout, phonetic pronunciation guides when necessary, and many examples and quotations. Click on the letter and scroll for the word.