This is my favorite of The Who's non-Tommy songs. I like all the different things going on here musically (particularly interesting when listening with headphones).
MTV's page about The Who tells why they smashed guitars:
...completely by accident, at a gig at the Railway Hotel, Townshend smashed
his first guitar. It happened by accident, because of a temporary stage
extension that the band had built, which was higher than the stage
itself, and caused him to accidentally hit the ceiling with his
instrument -- frustrated by his damaging of the instrument, and the
crowd's reaction, he struck it again, and again, and soon it was in
pieces, and it was only by using a 12-string Rickenbacker that he'd
recently gotten that Townshend was able to finish the show. The
following week, he discovered that people had heard about this, and had
come to the Railway Hotel to see him smash his guitar. He eventually
obliged with encouragement from Keith Moon, who attacked his drum kit --
and while Lambert and Stamp were at first appalled, Townshend smashed
another guitar to pieces a little bit later with Lambert's
encouragement, as part of his publicity campaign (and it worked, despite
the fact that the journalist for whose benefit he committed the
destruction never actually saw it). In reality, he didn't smash guitars
at every show in those days, and what he was doing, in terms of
generating feedback, sufficed in most audience's minds -- smashing the
guitar, when it did take place, only punctuated the feedback...
- The Who's website
- Songfacts has a long discussion of the lyrics
- Who are You was one of the Who's biggest U.S. hits (getting up to #14).