word choice - What's the difference between ようこそ and いらっしゃいま せ?

Both "ようこそ" (y­ōkoso) and "いらっしゃいませ" (irasshaimase) seem to mean "welcome" but what are the precise circumstances under which each should be used?

• "いらっしゃいませ" (irasshaimase) is uttered in unison by all the staff whenever you walk into a restaurant or shop


• "ようこそ" (y­ōkoso) was used in a recent "Welcome to Japan" advertising campaign

But this is just based on my very limited experience so I'm obviously missing lots of subtleties and insights.