Think of a workout as a state (of being). There has to be a way to enter; this is called the "Warm-up." The goal is to transition your body into the workout by increasing your breathing rate, blood circulation, warming your muscles and priming your body to get the most out of your workout. This phase should take 5-10 minutes and emphasize moving the large muscles of your body and gradually increasing the blood flow throughout. Typically, it consists of light cardio/aerobic movements such as walking or using cardio machines (elliptical, stationary bike, rowing machine etc.) as well as dynamic- moving- flexibility exercises such as toe touches, goblet squats, lunges, overhead reaches and movements that mimic what will be done in the workout itself. It can be tempting, when schedules are tight, to skip a warm up and jump right into the workout. However, warming up allows you to move optimally and intentionally; meaning you get more bang for your buck!
Conversely, a cool down follows the workout and is a way to transition your body out of that fight or flight state. This is a time to gradually slow your movement and breathing by easing into a gentle, cooler state of being. The purpose of a cool-down is to prevent dramatic drops in blood pressure, that can cause light headedness, and blood pooling in your lower body. Flexibility can be best optimized here because the muscles are warm and pliable. Rather than dynamic, moving stretches, cool downs prioritize static stretches where you hold a position at the point of tension for 10-30 seconds at a time. There is also some evidence indicating that it can lessen feelings of soreness after a particularly demanding workout. Even though you may be tired and ready to move on to the next part of your day, never skip a cool down!