Academy of American Poets
Poets.org: A Brief Guide to Concrete Poetry
Concise explanation of the term "concrete poetry." It's mainly a discussion of the 1960s poetic movement, but it also includes the history of the term before and after.
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.