Mexican cooking authority Diana Kennedy has said that the Serrano chile has the shape of a bullet. One could say that it tastes like one too! Serranos are spicy. However, as with most chiles, you can pump down the heat by removing the seeds and veins.
They have, like the Jalapeños, a dark and deep green color, shinny skin and a small and thin stem. However, Serranos tend to be on the smaller side and are much thinner and appear longer.
It seems to me, Serranos have a fuller and more flagrant flavor than the Jalapeños. Don’t buy them if the have wrinkled skin or brown or black spots.