Serrano chiles have a smooth skin, bright green color and very hot taste. They have slightly pointed tips and are smaller and slimmer than jalapeños.
Jalapeño chiles have smooth, medium-to-dark green skin that turns dark red when ripe. Jalapeños range from hot to very hot in taste. (Taste all chiles warily if you're a novice.) They have rounded tips and are about 2 to 2 1/2 inches long and 3/4 to 1 inch in diameter.
Jalapeño, serrano and other varieties of hot chiles contain the colorless irritant capsaicin, which gives them their hot bite. Direct contact with capsaicin can irritate skin and eyes. Wear rubber or plastic gloves (or slip a plastic bag over your hands) when handling hot peppers, and avoid touching your face or eyes. Scrub the knife and cutting board when you're fnished. If peppers do come in contact with your skin or eyes, wash the area well with soap and water or flush your eyes with running water.