Dave Shea has a nice tutorial on the web standards:
Its like this: your text is content. Content is nice, but without any hints about the contents structure (which includes things like spaces and headers and lists) you end up with a jumbled mess of text. Completely unusable. Structure is an extra layer which breaks down that messy text into logical groupings and organizes them in a way that conveys extra information about individual elements in that document. How that information looks may be implied by the structure (for example, youll most often find that a primary page heading will be larger than the body text) but it doesnt dictate it any further.
Thats where presentation comes in. Presentation is the formatting cue that tells the primary page header to be red, italicized, and 150% of the body copys size. Presentation is an extra layer of information on top of a documents structure, that builds up the (non-visual) structure into something far more appealing to the eye. CSS is the presentational layer, and it can take a very simpy marked up document, and turn it into something amazing.