A warm-up helps your body prepare its muscles and mind for both mental and physically exertions and reduces the chance of injury. The aim is to increase circulation, blood flow around the muscles of the body, in a gradual manner. During the warm-up any injury or illness you have can often be recognized, and further injury prevented.
Extra un-restrictive clothing (track suits - hats) can be worn, to help increase the body temperature, which in turn will speed up the warming up process of your bodies muscles.
The warm-up can be a combination of rhythmic exercise, such as those shown in aerobic videos, or a slower version of the aerobic activity to come. For example, you might want to walk before you jog, do the hoovering and mopping of the floor, prior to doing a light home circuit.
What is important is that you warm up and do some static stretching. The body will take time to warm up properly; i.e. shut done areas such as the digestive system, in order to supply the muscles with more blood.
Aim for a minimum of 5 minutes for a warm-up and the same for stretching. When working out, take into consideration that it may take a good 30 minutes for your body to be working at its best.
During cold weather, and certainly if you're going to do an activity which requires sudden bursts, make sure you spend more time warming up.
The following page will give you some sample exercises, suitable for a simple warm-up.