A variety of the Capsicum Annuum species. It name means in Spanish "from the mountains" because it was first grown in the mountains of northern Puebla in Mexico.
The Pods are typically 1 to 4 inches and maybe 1/2 inch wide and can be either erect or pendant. The pods start out as dark green and mature to a deep red. The Serrano is about twice as hot as a Jalapeno weighing in about 10,000 to 25,000 Scoville Units. Like most chiles they do not attain their full heat potential until fully ripe.
The Serrano is an excellent choice for the home gardener. They are hardy and very prolific, producing up to 50 pods per plant. They also highly recommended for use in "fresh salsas".