In broader sense, Waka (和歌) can be either Tanka (短歌 lit. short poem) or Choka (長歌 lit. long poem). Waka, though, usually means Tanka.
Tanka is a form of Japanese poetry, an even older form than haiku. It consists of five lines with a pattern of syllables in each line as follows: 5-7-5-7-7. To be exact, it is contained in waka, which consist of tanka, choka, etc. But usually, waka is equal to tanka. Each line contains a separate image but a unifying image runs through the entire poem.
There were selected poets called Six best Waka poets and Thirty-six best Waka poets.