SKU - Stock keeping unit, is an industry generic term to identify an item for sale such as a product or service. This code contains information about the item's size, color, material, manufacturer, etc. The primary scope of this code is to help businesses track their inventory. The format of a SKU consists of numbers and letters and with hyphens in-between, but can be only numbers or only letters as well. The following are examples of a SKU: tshrt-red-sml, 1234-000-911, 999-red-amz.
ASIN - Amazon Standard Identification Number, it is an Amazon specific code and serves the same purpose the SKU does. You can think about it like the Amazon SKU. You can customize the ASIN if you want, but you can also let Amazon automatically assign ASIN codes to your products. This code helps Amazon keep track of the merchandise. An ASIN code consists of unique block of 10 letters and/or numbers.
MPN - Manufacturer Part Number, is created by the manufacturer when a new product is created. This is used to track, organize and communicate the product's information internally and with 3rd parties.
Other tracking units like UPC - Universal Product Code, EAN - International Article Number, APN - Australian Product Number are special cases of GTIN. GTIN - Global Trade Item Number are standard, global, tracking units.
You can read more about these tracking code in our blog post here.