Weight Gain refers to increasing your weight. Weight gain may be involuntary due to over indulgence, stopping regular exercise, pregnancy, increasing alcohol intake or gradually increasing the amount of calories you consume.
Weight Gain also refers to people who are looking to gain weight though exercise - this type of weight gain is through building muscle mass. Weight Gain through weight training is quite different in that though the body may weight more, this is due to increased muscle not fat. Muscle is a heavier substance than Fat and most body builders have a very low percentage of body fat. Weight Gain through building muscle actually increases the rate your body burns fat and through weight training (anaerobic exercise) you will burn fat most efficiently.
Weight Gain products are available for those looking to 'bulk up' and increase muscle mass. Weight Gain products which aid increasing muscle mass are available from many high street stores or online.