A genre of rock that is more about the message than the music. Punk songs are usually, but not always, made up of 3 chords.
If you're not sure if a song is punk, listen to the lyrics. If they are not about standing up for something the artist believes in (ie: anti-censorship, anti-war, anti-racism, etc.) then it is NOT a punk song. Punk is about standing up and fighting for something you believe in. It has nothing to do with the image or the artist, or even how the melody of the song sounds.
Examples of punk bands:
Sex Pistols, Ramones, Clash, NOFX, Rancid, Bad Religion
Nonexamples of punk bands:
New Found Glory, Good Charlotte, Simple Plan, MxPx, Ataris, Thursday
Wannabe: "Hey! I wear clothes from Hot Topic! I'm punk!"
Real punk: "No, you're a poser. Punk has nothing to do w/ the clothes you wear."