GTIN stands for Global Trade Identification Number. This refers to any barcode that is used in the Global Retail System. So it can refer to a variety of different barcodes. I have detailed the most common of these below.
Please see the differences between EAN-13 and UPC-A barcodes here.
GTIN-14 – This refers to ITF-14 or TUN Carton Codes. These are used for cartons of 6 or 12 (or any number) of your product and is used purely for stock tracking by the company. It is based on a UPC-A or EAN-13 barcode, so the ITF-14 on a carton corresponds to the EAN-13 product inside the box.