a "Menü" is normally a complete meal. So, it could be salad or soup, manin dish and dessert. Or only salad or soup and main dish/ main dish and dessert. Mostly it is offered for lunch (at least in Berlin). Or in more expensive restaurants also for diner (and it will cost a lot)

"Menü" has two meanings: As choca said, it can be a pre-selected sequence of dishes or choice of dishes with one price for the whole meal. It is also a somewhat fancy and old-fashioned synomym for "Speisekarte". So in a high-class restaurant, you could say "Ich hätte gerne das Menü" instead of "Ich hätte gerne die Speisekarte". If you said "Ich hätte gerne das Mittagsmenü, und zwar als Vorspeise die Suppe und als Nachtisch das Eis" or "Ich würde gerne das Menü Nummer 3 für zwei Personen bestellen" then you already have the Speisekarte in front of you, and you are ordering a particular choice of dishes.