The first three lines are usually as concise and evocative as a haiku, and tend to set the scene through physical description (although there are exceptions, of course). These opening lines do not usually involve metaphor or simile (or, if you prefer, any sense of comparison).
The last two lines expand, comment, and/or delve more deeply into the emotional heart of the poem. This is where thoughts and feelings enter the tanka (usually), and the use of metaphor or simile is highly encouraged here. The goal of a tanka isn't just to present the reader with an image, but to deliberately evoke a feeling or emotion, whether it's pathos or humor or something else.
Here is an example from Carl Brennan, a New York poet who won grand prize in a tanka contest with this one, which many writers can relate to:
She comes at night,
wrapped in convulsive perfumes
and scarred by longings,
lavishing the wrong names on me
at the worst moments: my muse
The first three lines describe a woman in interesting imagery, and we get the sense that this woman is tortured (and possibly spreads that torture around). The last two take the situation - this woman - and makes it personal: she affects ME and is so inconvenient, before identifying that she isn't necessarily a real being at all, but is instead the urge to write. Love it.
Tomorrow, a look at Ono no Komachi, a historical figure and one of the most famous female Japanese poets and a complete master of the tanka.