Jeffrey Zeldman: King of Web Standards

CSS allowed developers to separate content from appearance; style sheets are like little notes that say to the Web server, “If youre sending a page to a PC, make it look like this.” There might be separate sheets for PCs, for a “printer-friendly” layout, for a PDA, and so on. For designers, CSS means that the page will appear as it was intended, no matter what the device. For developers, CSS means they only have to build the page once. And for users, CSS means, as Zeldman says, that the site works.

There’s a bit more info on this over at Zeldman.com