Set references are the usually-unique numbers found on the majority of LEGO numbers. They come in two types, usually but not exclusively referred to as product numbers and item numbers.
The product number is the number usually seen on the front of the product or box and is usually very clear to see. It is also referred to as a set reference, set number or item number. These are usually exclusive to the set, though many before #1000 may refer to two or more sets. These are used for easy identification of products and benefit both the supplier and the customer. They only appear on sets and some merchandise. For a list of known product numbers, see List of Set References.
The item number is usually a 7-digit number that appears on the back of a product or above or below the bar code. Unlike the product number, these are all individual and are found on sets, merchandise and instruction booklets. Parts also have these numbers. They are usually in the 4xxxxxx range, although in recent years they are starting to begin with a 5 or 6. LEGO Education US also use item numbers that are 9-digits long. For an (incomplete) list of item numbers, see List of item numbers.