Sweating is your body's mechanism to cool itself. Your nervous system automatically triggers your sweat glands when your body temperature rises. Sweating also normally occurs, especially on your palms, when you're nervous.
In hyperhidrosis the nerves responsible for triggering your sweat glands become overactive and call for more perspiration even when it's not needed. With stress or nervousness, the problem becomes even worse.
Often there is no underlying cause of heavy sweating; this condition is called primary hyperhidrosis. This type usually affects your palms and soles and sometimes your face. It may have a hereditary component, because it sometimes clusters in families.
If the sweating can be attributed to an underlying medical condition, it's called secondary hyperhidrosis. This type is more likely to cause sweating all over your body.