A galvanized trailer is usually dipped and coated with a layer of zinc (the whole thing!). The zinc provides really good rust protection (better, usually, than paint). It will be a good bit heavier than an aluminum trailer and you'll really notice the biggest difference while moving it around by hand with a boat on it. You might notice some increased efficiency while towing with the aluminum trailer, but my experience has been that the mileage difference is pretty tiny.

Are you buying a new boat? They should be able to tell you what trailer it comes with - often, however, trailers are optional items.