What exactly is a sonnet?

Ok so I like thisd girl right? I plan on sending her an annynomus carnation / flower to her home base with candy attatched to it right? she knows i like her and my friend told her that is she recieves one from a secret admirer its me. and she got all excited like "omg!!!!! :) tell him to attatch candy too it with a sonnet..... whats a sonnet??? It kinda sounds like a poem to me but now that i think of it i plan on attatching the lyrics too one of my favorite teen love songs on it "there is" by: box car racer.... Would that not be a good idea? i could prolly write one right now if that would be better..... but anyways lets get back to the basics.... whats a sonnet?...