[Torchwatch]

A G string (or posing pouch) has a pouch front, a strip of elastic waist band, and a strip of elastic rear, that passes through the ass crack. The elastic may be replaces with actual string, but this has to be tied to the correct tension.

A thong has a pouch front and a much reduced fabric rear that passes though the ass crack. The waist elastic and 2 leg elastic loops are built into the fabric of the suit. A thong is come comfortable than a G string as it doesn't have a cheese wire effect on rear of the wearer.

A string bikini is a side-less swimsuit with a fabric front and back held together with an elastic or string waist band. The rear may vary from a Brazilian to a full cut, but must cover the ass crack (or it would be a thong), although a low cut string bikini may expose the top of the ass crack.
String bikinis using string need to have the side strings tied to the correct tension, while an elastic string bikini may have the elastic sides enclosed in fabric for a good finish.
Some brands use assorted hooks etc to join the sides and decorate the suit.
The underwear version of a string bikini is sometimes called a Tanga.

The more one attempts to define swimwear the more difficult it becomes due to regional and language variations.