"Sour" is never a good thing, well unless you're making a sour, on purpose, of course!
If it's "tart", like green apples, that could mean just a young beer (it's called acetaldehyde) because it's an intermediate compound in the formation of alcohol. Some yeast strains produce more than others, so sometimes it's worse than other times or even absent. But generally its presence indicates that the beer is too young and needs more time to condition. It's definitely a green apple smell and flavor, though- not really what I would call "sour". Sour taste tends to mean infection, but hopefully it's not the case here.