Jeffrey Zeldman:

They mistake Style for Design, when the two things are not the same at all. Design communicates on every level. It tells you where you are, cues you to what you can do, and facilitates the doing. Style is tautological; it communicates stylishness. In visual terms, style is an aspect of design; in commercial terms, style can communicate brand attributes.

Originally written in 1999, updated in 2005 — no joke.