Short answer
A GTIN identifies a trade item. GTIN-8, GTIN-12, GTIN-13 and GTIN-14 differ by length and use; EAN-13 and UPC-A are common forms.
GTIN is the global standard
GS1 manages the identification system so systems can recognize the same trade item.
EAN and UPC are common forms
EAN-13 is common in Europe and UPC-A in North America, and both belong to the GTIN family.
Length has a purpose
GTIN-8 is compact, GTIN-12 and GTIN-13 commonly identify consumer items, and GTIN-14 often identifies packaging levels.
The barcode is not just the number
The GTIN is the numeric identifier; the bars are its machine-readable representation.
