The main difference between glasses and goggles are the way that they are held on your head. A pair of goggles will be full seal, and will hold onto your head by way of an elastic retention band. A pair of glasses might not be entirely full seal, and, while it may have a retaining cord to help keep the glasses in place, is mainly held on your head thanks to the temple bars hooking behind your ear. Which type you choose to wear is based on personal preference, as well as which sport you are playing, and the the hazards that you may encounter.