Belgian/blonde/hefeweisse beers

Weissbier means "white beer" in german. It's a traditionally german style of beer. It's brewed with wheat and malted barley. As mentioned before, the banana flavors come from the combination of wheat and the type of yeasts used (high fermentation yeasts, btw). They are not filtered and, because of that, cloudy. Filtered Weissbiers are known as Kristallweizen.
On the other hand, Belgians produce Witbiers which also means "white beer", but in Flemish (a variation of Dutch spoken in the north of Belgium). Witbiers are also produced with wheat. They are also hazy BUT, they are quite different in taste and smell from the German Weissbiers. Witbiers are traditionally spiced with orange peel and coriander and do not have banana flavors. I suppose that they use different yeasts strains.

 

Yep, it's from the yeast... on both counts. As the beer ferments the yeast strain produces fruity esters that we percieve as banana flavor. It also not very flocculant so it stays in suspension and creates the signature cloudiness. The wheat also contributes to the cloudiness.