The Japanese place more emphasis on the imaginary used and the emotions evoked by Tanka than they do on the structure of the poems. Therefore, the structure is not nearly as rigid as, for example, that of a Sonnet.
Often Tanka and Haiku are mistaken to be the same. No doubt they are very similar for a westerner, but there are certain differences that are worth noticing.

Tanka is the most prevalent verse form in traditional Japanese literature. In fact, for many centuries the Tanka was virtually the only form used by poets who wrote in the Japanese language. Each short poem consists of five lines of five, seven, five, seven, and seven syllables. The enduring popularity of the Tanka form resulted partly from the limiting characteristics of the Japanese language, which made it difficult to maintin a high level of intenstity throughout a long poem, and from the Japanese preferences for simplicity, suggestions and irregularity, which are reflected in the brevity of the Tanka form, the evocativeness of the poems, and the uneven number of lines and syllables per line. Most Tanka include at least one caesura (or pause), often indicated in by English by punctuation.
Used as a means of communication in ancient Japanese society, Tanka often tell a brief story or express a single thought or insight. The most common subjects in Tanka are love and nature. In expressing their feelings of love or their appreciation for nature, poets often show restrain when exhibiting their emotions, they rely on clear and powerful images to evoke an emotional response rather than using abstract words to directly express their feelings. At the same time, Tanka poets often hint at or suggest the existence of a higher reality. Take for example the following Tanka by Oshikochi Mitsune:

At the great sky
I gaze all my life:
For the rushing wind,
Though it howls as it goes,
Can never be seen.

The Japanese place more emphasis on the imaginary used and the emotions evoked by Tanka than they do on the structure of the poems. Therefore, the structure is not nearly as rigid as, for example, that of a Sonnet.
Often Tanka and Haiku are mistaken to be the same. No doubt they are very similar for a westerner, but there are certain differences that are worth noticing.
The Haiku, which consists traditionally of three lines of five, seven and five syllables, evolved from a form of collaborative poetry known as renga. Consisting of chains of interlocking verses of seventeen and fourteen syllables composed by groups of poets, the Renga form thrived during the medieval age. Eventually the hokku, the openning verse of a renga, developed into a distinct literary form known as Haiku.
Reflecting the dominant tastes of the Japanese culture, Haiku is characterized by precision, simplicity and suggestiveness (similarly to Tanka). Hakai (pl. for Haiku) present spare yet clear images that stimulate thought and evoke emotion. Because of it's brevity, the images cannot be presented in detail. As a result, Haikai employ the power of the suggestion to produce detailed pictures in the reader's mind. For example, most Haikai include a kigo, a seasonal word, such as "rain" or "cherry blossoms" for example; and in doing so they indicate the time of year being described in the poem, but they also evoke the reader's reminiscence of that time of year. Although some Haikai seem to contain one single image, most of them present an explicit or implicit comparison between two images, actions or states of being. This is a capital feature of Haiku that sepparates it from Tanka.
Take for example the following Haiku by Basho:

Poverty's child-
He starts to grind the rice
and gazes at the moon.

By contrasting the task of grinding rice with the boy's observation of the moon, Basho evokes a sense of longing and captures the soothing effect of nature on the human spirit. We can say that in general terms Haikai present an image of nature that combines with one of a different kind, oftenly one related to life or the meaning of life:

Summer grasses-
All that remains
of soldier's visions.
Basho

In this case the reference to summer and to the general splendor of this season has two purposes: to evoke an emotion in the reader's mind and, when combined with the other image presented, to suggest the fleeting nature of life: a soldier's visions of the sweetness of summer (perhaps whilst marching to battle or just before meeting his death). By contrasting these two images, Basho evokes in us a sense of longing for our own personal "summer grasses" and for the memories that mean the most in our lives.
We could say therefore that Haiku is often a strongly philosophical form aside from a refined lyrical one.
Sometimes, Haikai creat a constrast by rapidly shifting their focus from the general to the specific or viceversa:

An old pond:
a frog jumps in-
the sound of water.

Because of it's brevity and suggestiveness, Haiku demand extra effort on the reader's part. They demand imagination.....