Most books about Punk have focused on a few bands and the "scene", but that's not really what Punk is. Yes, the music is an important part, but really it is a philosophy and a way of approaching and relating to the world around us. As a result, musically Punk has often been a dumping ground for whatever isn't easily categorized. At times, The Ramones, Sex Pistols, Blondie, Talking Heads, Joe Jackson and Brian Eno have all been considered Punk. Hell, even the first Huey lewis and the News album is a Punk album. After time passes and others adopt the musical style of a Punk performer, it gets its own name and is reclassified. That doesn't mean it isn't still Punk.
New Wave, Goth, Ska and Emo are all offshoots of Punk. Some have become their own genres, others are still considered Sub-Genres of Punk. Punk has been able to grow enormously and expand in many directions and yet still be considered Punk. Musically, this makes a definition difficult. Originally many consider Punk to have been a reaction to what was happening to Rock & Roll in the late 60s and in the 70s. Rock was becoming more florid, more masturbatory. Performers seemed to feel a 20 minute song full of meandering guitar or keyboard solos made good music rather than being narcissistic posturing like children demanding attention, yelling "look what I can do". Punk at it's base called back to early Rock & Roll where the melody was key, where the song was short, basic and simple. This is partly accurate, but there are problems with the definition. Stiff Little Fingers and others had a fondness for the guitar solo and many used complex melody and harmony in their work. Reducing Punk, even just the musical portion, to this basic definition was diminishing the scope and importance of the movement.
While Punk can be about sex, it isn't to the same degree that most other music seems to be. Punk, when it deals with relationships at all, tends toward longing, love, unrequited love, feelings of inadequacy, rejection, hope and compatibility. In other words, Punk deals with sex like it deals with all other subjects, as something complex and urgent. Punk is often protest music, even if that protest is on a personal level. Punk is outsider music, by and for people that don't see the world as a shiny happy place where you get what you want and nothing really matters in the long run.
Another aspect of Punk, and one that is true both of its music and philosophy, is the Do It Yourself ideal. Some of this is about being self-reliant and competent. Some is about refusing to let others decide things for you, but much of it is about being able to do for yourself. Anyone can create music, not just those capable of technically complex instrumentality. The Ramones are one of the pioneers of this, and inspired hundreds of other punk bands; bands that likely never would have existed without The Ramones showing that it could be done.
This brings us to the non-musical aspect of Punk. The Philosophy, generally speaking, is one of independence. If you don't fit in and don't care, or even think it is a good thing, that's Punk. If you are, to borrow from the "new agey" people for a moment, "true to yourself" then that's Punk. However, there's also no apology for being this way. There's no I'm OK, You're OK. It is far more a philosophy of "I'm OK, Fuck You." This doesn't mean not caring for or about others, just not caring about judging others and what others judgments are of you. It is about defying and questioning authority, musically and otherwise. It isn't really about anarchy, but that does make a good slogan.
In the end, Punk defies definition. I know a Punk song when I hear it, but maybe I miss a few. Sometimes it is when the band thinks they're Punk, sometimes they don't think so, but they are and sometimes they think they are, but really don't understand what Punk is. If you want to know if a specific band or song is Punk, decide for yourself. If that doesn't work, buy me a copy and send it to me and I'll tell you.
When I'm down
I just play guitar real loud
Crank the Pistols and Ramones
Drive the neighbors from their homes
-- Social Misfits - Atomic Boy