Original: A G-string (alternatively gee-string or gee string) is a type of  underwear, a narrow piece of cloth, leather, or plastic, that covers or holds the genitals, passes between the buttocks, and is attached to a band around the hips, worn as  swimwear or  underwear by both men and women.
Simplified: A G-string or thong is a narrow piece of cloth or leather, or plastic that passes between the buttocks. It can be worn as a swimwear or  underwear by both men and women.