If a haiku is usually (mistakenly) thought of as a 3-line, 5-7-5 syllable poem, then the tanka would be a 5-line, 5-7-5-7-7 syllable poem. However, as with haiku, it's better to think of a tanka as a 5-line poem with 3 short lines (lines 2, 4, 5) and 2 very short lines (lines 1 and 3).
While imagery is still important in tanka, the form is a little more conversational than haiku at times. It also allows for the use of poetic devices such as metaphor and personification (2 big haiku no-no's).
Like haiku, tanka is a Japanese poetic form.
While I'm sure there are problems with my attempt, here is my tanka attempt, which you can use as an example of the form:
turn circles in my head
as I think of her hand
turning the world inside out
Here are some other online tanka resources: