Simply put, their base contact points. In other words, a 1156/3156 is a single filament bulb therefore used only in applications where one filament would be necessary i.e. reverse lights, running lights, etc. Whereas a 1157/3157 contains two filaments. These are most commonly used in turn signals where two filaments are needed. In order to have an LED turn signal, you must use resistors to allow a complete circuit or you will experience “hyper flash” as mentioned above.