How do I understand how top sites and web apps actually function underneath the hood?

I'd like to get into web application development, but I'm pretty much a novice. How do I even begin to guess/understand how sites such as Twitter or say, HipMunk, might work in the first place, top to bottom?

This link does an excellent job of Quora's 开发者_开发技巧underlying technology: http://www.philwhln.com/quoras-technology-examined

Any links or pointers that I could use would be immensely helpful.

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Look here: http://highscalability.com/

Specifically, browse this section for real-life examples of architecture of highly loaded web-sites.