Newport Beach, CA, USA
Thanks Nocht...short answer...a tanka is a kind of extended haiku...but really haiku is an offspring of tanka. The Japanese long ago would have ritual renga competitions....a renga being a poem composed verse by verse by several poets together, each adding a verse to the last. The verses were called tanka...each a poem in itself but also a part of the whole renga. tankas were connected by haiku, which eventually became a form in its own right.
The form is 5-7-5-7-7.