Bottom Line: The Serrano chili is hotter than the better-known Jalapeno with more taste and can be used in a wide variety of dishes or as a topping by itself.
While the Jalapeno is the most-recognized chili pepper worldwide, and the superhot pods like the Ghost Pepper and Carolina Reaper get a lot of attention these days, the Serrano chili is a major contender as well, and one that I personally reach for quite often. They may not be as well known, but they are well-traveled and according to Wikipedia they are the second most-used chili pepper in Mexican cuisine after the Jalapeno.
The Serrano is hotter than the Jalapeno, typically coming in between 10,000 and 25,000 Scoville Units (and some can be hotter) vs 4,000 and 8,500 SHU for the better-known pod. I have these at the top end of Medium on my Heat Scale, and I find them to be a tastier chili, with a nice brightness to them and a hint of berries as opposed to the Jalapeno which tastes more like a Bell Pepper with some heat added. They are quite common in the markets here in Texas, and you usually find the green ones in the produce section (the more mature pods, which are red, are rarely seen in stores). When picking out Serranos, you want a nice, firm chili and typically the smaller ones will pack more heat. The Serranos I find at the store almost always have good heat as opposed to Jalapenos which sometimes have no burn at all. (There actually is a Nadapeno hybrid which is a milder version of the Jalapeno and those seem to have crept into the markets.)
The Serrano is my standard go-to chili for adding heat to a dish. It won’t make it super hot if you do not add too many, but it will kick things up better than a Jalapeno. The Serrano is a must-add to pico de gallo (recipe at this link), and I use it in most of my salsas. I also tend to just dice up one of the peppers and put that on top of tacos, enchiladas, burgers, and more. Roasting Serranos on the grill or in the oven is also a great way to add flavor to a salsa or a dish while also kicking up the heat (more on that here).
The Serrano originated in the Mexican states of Puebla and Hidalgo, and its name is derived from the Sierra mountains in those regions. These days, the states of Veracruz, Sinaloa, Nayarit, and Tamaulipas produce about 180,000 tons of the spicy pods on a yearly basis (and us chiliheads salute their hard work). The Serrano is not quite as well known as the Jalapeno, but it is almost always available in the stores in Texas. It is high in Vitamin C and low in calories, and it is a great way to spice up food without adding sodium or packing on the pounds.