Though I disagree with summing up bands as a particular genre, we all have a vague idea of what each genre means... So, what's that? Cite bands, humans and record labels...
Do add more genres.

rock - Loud, guitar-led. Early examples include You Really Got Me and Paranoid, and these days the most quintessential rock bands are probably Queens of the Stone Age and Foo Fighters.
punk - Again loud and guitar-led but less focus on proficiency, more focus on defiance. Created to counter prog w*nkery, and (here comes a cliché) far more of a state of mind than a sound. 'Pop-punk' is not punk. The Sex Pistols, Nirvana and Rancid are.
pop - The familiar singable structure of a modern song, which applies to most popular music. Music which has no shame in trying to be catchy. Not necessarily factory-line crap. The Beatles, Supergrass, Annie, Girls Aloud.
indie - A dead term for alternative music which actually referred to record labels and could be applied to (among many others of varying sound) The Smiths, now applied to everything with guitars not on their highest setting (ie, everyone who's a fan of The Smiths or at least pretends to be). The ignorance and accents assosciated with the modern definition makes it generally a derogotory term in my mind (Scouting For Girls, The Pigeon Detectives), but this isn't necessarily the way (The Cribs).

prog - musical masturbation.

Dance: Anything with lyrics that are repeated all over the songs, and random bouts of upping the tempo for no reason. Commonly found in Ibiza.

Trance: Dance, but with no lyrics beyond a brief hint of chorus at best.

Emo: Take Colin's entry for Punk, but get rid of any lyrics about politics or defiance of culture, edit it to sound completely pre-recorded, and add lyrics about how hard it is to be in a relationship with someone for any given reason. Also add a few wailing "ooh's" for no discernible reason.

Pop-Punk: Emo, but without the lyrics about relationships. Also not about politics, or defiance, and general just about anything that won't inspire rebellion, or make people think about love. Less wailing in older Pop-Punk.

Little Blue Fox. wrote on Aug 12^th, 2008, 3:51pm:


hahaha

Dr Namgge wrote on Aug 12^th, 2008, 1:02am:

Not listened to Green Day recently, have you? (They're pop punk, always have been, not in the obvious sense, but when compared to punk as a whole its pretty apparent.)

Hmmm, I don't really think NOFX can be called 'pop' punk.........

Really? i find it quite poppy.

the doc wrote on Aug 12^th, 2008, 4:59pm:

I'm in agree-ance.


See to me pop punk would be the way Green Day sound currently, or the back end of the Offspring career. NOFX were always harder hitting than these guys throughout.