Thong

A thong is a type of underwear, in which a V-shaped piece of cloth covers the genitals, then passing between the buttocks as a narrow band of fabric, finally rising up to the lower back and connecting to another V-shaped piece of cloth in the back.
Shown below: back side facing user

V-String

A V-string is a thong that is cut in a distinct V-shape; the waistband rises sharply upwards in a V-shape, so that it sits further above the hips.
Shown below: front side facing user

G-String

A G-string is a type of thong or underwear, in which a V-shaped piece of cloth covers the genitals, then passing between the buttocks in the form of a string, finally rising up to the lower back and connecting to another V-shaped piece of cloth in the back.
Shown below: front side facing user

T-Back / Micro G-String

A T-back / micro G-String is the same as a G-string, except without the V-shaped piece of cloth in the back.
Shown below: front side facing user

C-String

The C-String is as narrow as a G-string, but without the supporting "string" around the wearer's hips/panty line, leaving just a sideways C shaped piece between the legs. It is held in place firmly by a flexible internal frame. Since there is no material around the waist, the C-string completely eliminates the panty lines which thongs and other underwear create. C-strings are also designed for use as beachwear, which reduces the tan lines that would have been left by the side straps of even a G or V-String. A C-string is so named because it looks like a C when viewed from the side.
Shown below: back side facing user (the wide side is the front)

Cheeky

A more conservative style of thong called a cheeky covers a little more area, but exposes the bottom part of the buttocks. Cheeky panties are slightly bigger than thongs, but are slightly smaller than normal panties. Some cheekies are used as undergarments while others function as bikini bottoms. Often they have a band at the waist.
Shown below: back side facing user