The best before date, is usually a measure of quality, meaning taste, texture, aroma and appearance. Specifically for foods that do not support the growth of bacteria and other micro-organisms. This means that the product being consumed has a measure of when the best quality of the product would be. Such an example would be ice-cream, butter and breads. These also include tinned and dried products.
The use by dates are significantly different. This date is a measure of suitability for human consumption, and is measured on a scientific basis. These dates would be present in foods that support the growth of bacteria, and include all fresh and ready to eat foods.
There are a number of factors that can determine both dates, including the addition of preservatives, pH, salt content etc. These are the conditions that support or prevent the growth of bacteria.
Check out our article on best before dates