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.

well a 1157 outs out about 412 lumens, a 3 watt led (luxeon) puts out about 120 if its lucky, so the incandescent bulb is still 3x bright. so go buy new incandescent. i know its cool to say you have led lights but most of them dont come close. these ones are the only ones that come close to regular bulbs the high number smd leds.

When replacing old incandescent bulbs with LEDs, a common question that customers ask is: “Can I use an LEDs bulb that has a higher watt equivalent than my fixture allows?” The answer is yes if the light bulb uses less power than the fixture. See also 9 Best LED Lights For Dyna Can LED bulbs catch fire?