Meaning and Commentary
From a now-gone writeup: "For whatever reason, pedophiles are some of the most feared and hated people on the planet."
From another, I think: "someone who is more interested in jacking off to a picture of a naked kid than wondering "where did that picture come from.""
A pedophile is somebody afflicted with pedophilia. That is to say, they are sexually attracted to children. They don't necessarily view or support child-porn or go around having sex with minors. Do you have sex with everybody you find attractive? No? Me neither. So why do you assume they're any different?
It's been pointed out to me that paedophile often specifically means a child molester--perhaps more so in Britain than the USA. This, however, may be in part because who's going to say 'Oh, yeah, I'm sexually attracted to children but keep my sick urges under control'? Not many people, so the only paedophiles you hear about are people who tried to have sex with a minor or viewed/made child pornography and were caught, and the word remains with strong de facto connotations regarding actual behaviour, creating a positive feedback loop. Whether this significantly deters paedophiles from getting psychological help, I can't really say, but it probably does to some extent. It probably also makes it more apparent that their condition is a problem, though.
Etymology and Spelling
pedo- is the Americanised prefix for child, the British being 'Paedo'. 'Pedo-' (sometimes 'pedi-') also means feet.
It should also be noted that 'philia' is non-sexual love--brotherly love. Romantic love is 'eros' so the correct term is--I've been told--is paedorast. (Update: Uri E Bakay tells me that in Lolita, Humbert calls his condition 'pederosis'. '...but of course, that's the american spelling and there should really be an a in there.' Of course there should. Silly Americans.)
In Britain, there have been cases of paediatricians (doctors who specialise in treating children) being targeted by violent mobs due to idiots' seeing the prefix 'paedo' and thinking 'OMG, paedophile!' Silly Brits. WikiMedia (of Wikipedia fame) was considering the name 'PediaWiki' for their software, this was mentioned as a potential problem. I don't know how seriously they took that issue, but in the end, they did settle for MediaWiki instead of PediaWiki.