www hacking?

I havent seen much in lines of this being a problem yet..although I was reading through practical unix security (newest edition) and it made mention of the danger of hacking WWW. Forgive my ignorance on this.. but what dangers are there? I could see someone attacking the main server and do damage in that sense..but Im a little outdated in this as far as what is attacked. The only thing Ive ever had to worry about was unix, now theres oh-so much more. If someone could tell me the dangers of someone attacking someones site..etc..on the WWW, I would much appreciate it! Thanks Adam