The difference between Thong and Underpants

When used as nouns, thong means a strip of leather, whereas underpants means underwear, clothing covering the genitalia.

check bellow for the other definitions of Thong and Underpants

Thong as a noun:

A strip of leather.
Thong as a noun (usually, in the plural, Australia, US):

An item of footwear, usually of rubber, secured by two straps which join to pass between the big toe and its neighbour.
Thong as a noun (UK, US, New Zealand):

An undergarment or swimwear consisting of very narrow strips designed to cover just the genitals and nothing more.

Examples:

"No! I won't buy you a thong. You're too young for that."
Thong as a noun:

The largest section of a bullwhip constructed of many straps of braided leather.