A bullet shaped pepper. If you’re familiar with jalapeños then you already have a good idea of how serranos taste. Most people won’t be able to tell the difference in flavor — serranos will be sharper and have a more grassy flavor.
As chili peppers get smaller they tend to get hotter, and serranos will typically be about three times as hot as their jalapeno brethren. In other words, they will have a capsaicin concentration that’s about three times higher than the jalapeno.
They are meaty peppers and are not the best choice for drying, though it can be done. The serrano pepper originated in the Mexican states of Puebla and Hidalgo, in the mountainous regions. The name of the pepper, serrano, actually is a reference to the mountains (sierras) of those areas.
When you see a recipe call for jalapenos you can typically substitute serranos and vice versa with no problem whatsoever.
Find out more abour serranos here.