Hacking is about more than knowing the innards of a computer; it's about knowing the innards of a computer, networks, routers, scripts, cookies and protocols, and a boatload of other things.
This link explains a lot about hacking: the history, schools of thought, and evolution are all very written. Part 2 explains, in detail, the layers of TCP/IP (OSI model) and the various protocols used.
Part 3 talks about various methods and tools that can be used for penetration testing (SQL injection, session hijacking, various software, etc.)
The article also has numerous links to other pages with related information.
The most important thing (and the most mentioned thing) that the article teaches is legality. To paraphrase: Don't hack in systems/networks you aren't authorized to, don't download warez in an attempt to hack, and (for PEN testers) never exceed the predetermined limits of an authorized attack.
N.B. The article is about the theory, not the application
, of hacking. (i.e., no step-by-step instructions on how to perform a specific hack)