The concept of sound design is hard to explain, as it can mean something different to everyone who explores it.
One thing's for sure, though: you're going to know a lot more about the subject once you've watched Eric Persing (Spectrasonics), Diego Stocco (Sherlock Holmes and numerous crazy sound design projects (opens in new tab)), Richard Devine (Schematic Records) and Scott Gershin (Gladiator, Star Trek) discussing their craft, philosophies and techniques at Moogfest 2012.
There's almost an hour's worth of conversation in this Mavericks of Sound Design clip, so expect plenty of valuable insight.