What is the definition of a poem?

I'm not really sure how to explain what poem is. Sometimes they rhyme sometimes they don't. Sometimes they help you understand life, or get through life. Music is poetry, well the lyric are. A lot of people use music to get through life. Of course poems can be about love. They can be about a guy who loves a girl or vise versa. Sometimes they are just to convey a message. Sometimes just to tell people how they are feeling, or a lesson the author feels people should know. Sometimes poetry is purely entertainment. Sometimes people use it to vent. My cousin wrote a ton of really sad poems about her mom. Her mom is really cool so I don't know why she wrote them.