A Sound Designer is the supervising sound person usually involved from the beginning of a films/TV shows conception to the very end of final mix. He works very tightly with the director to form a sonic universe that fits the directors vision. He is considered to be the top department head of the entire sound crew, and therefor is responsible for hiring various department heads including Production Sound Mixer, Sound editors, Foley artists, Re-recording mixers, etc.
Depending on the projects scale, the Sound Designer might take on one or more editing or mixing roles himself.
He works closely with the director and the composer and music superviser to make sure that the overall soundtrack has a consistent feel to it, and supervises the sound editors to create all the sounds that are necessary for the final mix.