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.
Play with poetic forms!
Poetic forms are fun poetic games, and this digital guide collects more than 100 poetic forms, including more established poetic forms (like sestinas and sonnets) and newer invented forms (like golden shovels and fibs).
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: