Well, as far as the average consumer is concerned, the generic definition of a modern summer fragrance tends to be one that is light, and aquatic. Go to a fragrance department in your average store and you'll find this to be most of what they show you all year (in my brief experience). If you're referring to the summer edition of a cologne (i.e. Armani Code Summer, CK One Summer) this is still usually the case. With few exceptions most summer editions are lighter, more aquatic versions of their original counterpart. Not all of them (aquatics or summers) are bad, but I haven't found one that really makes me stand up and pay attention yet.
To me, a summer scent is one that is simply pleasant smelling (necesarry for all scents really) and not so heavy that the heat causes it to choke everyone around me. You'll find that pretty much everyone has a different definition of a "summer scent."