Now that's a complicated question.
What does it look like? It is words written in short lines
(usually). Sometimes it is broken up into groups of lines
(stanzas). Sometimes it rhymes. Sometimes it has rhythm. It
very often has metaphors and similes in it. Frequently, it
is meant to be read aloud, or sung.
What does it do? It says something you couldn't say any other
way, just like a painting or a piece of music. Very often,
it says things the reader already knows about the world, but
can't put into words. Sometimes it makes you laugh. Sometimes
it makes you sad, or angry, or just thoughtful. Like music
and paintings, it is easy to know poetry when you see it,
but hard to explain. Everybody has an opinion about it, too,
and many of the opinions disagree.
Ask me if you're not sure whether something is a poem or
not, and I'll try to help you figure it out.