What is a Bar Code?
It can mean many different kinds of bar code formats. Most commonly it is known as a UPC/GTIN-12 (Universal Product Code) or internationally as the EAN/GTIN-13 (European Article Number).
The UPC is a unique 12-digit number & formula designed for use by a non-profit called the UCC (now known as GS1) in the early 70's. This UPC allows retailers to easily track sales of your product within their inventory system.
Later in the 70's in Europe the EAN was created & used by the EAN foundation in a similar fashion, this is a 13-digit number & format. The EAN foundation later merged globally with the UCC and is now known as GS1. (Global Standards One)
"GTIN" is a newer name given to the UPC/EAN codes by the GS1 organization, it means "Global Trade Identification Number", and the "dash" and number afterward equal the total digits of the code. ie: The UPC is 12 digits long, so it's a GTIN-12 and the EAN would be a GTIN-13.
Below is an example UPC/GTIN-12 bar code image commonly used in the USA & Canada.