Don't worry about looking at code for huge sites...too much stuff there.
And you don't have any problem with selectors...you just described how to use them! Yes, #wrapper p would only change paragraphs in the wrapper.
You could also have:
#wrapper a span {}
Which would only change spans that have anchor tags.
DIVs...use div's for positioning things. Say for a website framework, you put a wrapper, and then inside that a header div...then UNDER that your content div, then below that your footer div.