A thong-like undergarment, only the back is a thin string of cloth. A g-string is generally more revealing than a thong. The two are best distinguished by their side straps: a thong will usually have wider side straps than a g-string, hence the presence of the word string. A g-string will also generally sit higher on the wearer's hips than a thong.
This tag implicates thong (learn more).