There's no legal definition of 'low-fat' although, as a guideline, three to 20 per cent is considered a moderate amount of total fat in a food. By law, manufacturers' claims mustn't mislead consumers, but beware of high sugar content in some low-fat foods.
Products which state "no added sugar" on their labels are not necessarily sugar free. Although no additional sugar is added, they may contain sugars derived from one of their major components. For example, an orange juice drink will contain natural sugar from the orange juice ingredient. A 'sugar-free' product shouldn't contains more than 0.5g of sugar per serving (100g or 100ml).
So basically, 'light' versions or alternatives may contain less fat or sugar or less of both, but compared to what? Government guidelines state that the term 'reduced-calorie' should mean at least 25 per cent less calories than in the standard version. However, if the standard version contains 2 billion calories, the 'healthy' alternative isn't exactly going to be slimming!