There are many different styles of poetry, from serious and lengthy odes to silly and rude limericks. Some poets always write rhyming poetry, some write free verse non-rhyming poetry. Other poets, and I'm one of them, write in a variety of different poetic styles.
The chosen form of a poem may be dictated by the rules of a poetry competition, or it may be written in a particular form as a game to see if you can do it. One poetic form, the villanelle, is a tricky form that I have written in just to see if I could do it. I tried it and liked it, but it's not an easy game. Writing limericks is easier, and lots of silly fun too!
The villanelle poem that is best known to people is one by Dylan Thomas that begins 'Do not go gentle into that dark night...' You can read more of this poem and more about this form of poetry here.
If you are interested in exploring poetry and poetic forms some more, I suggest you go to this website and read about many of the forms on offer. Your will also learn about various terms that have a specific meaning when talking about poetry. Of course, if you truly wish to learn more about these different forms, you could try writing one, and learn about it from the inside out! You'll learn to respect more, other poets who write well in a structured poem.
A good poem can look simple, but it can be damned hard work making it look so easy.
I hope you are inspired to take a deeper look into the wonderful world of poetry!