The T's were a street gang in the future Gotham City.
While the Jokerz wore a wide variety of clown-themed costumes, the T's held to a very strict dress code: White body paint covering all visible parts of the body, with a bright-red "T" over the face. They also wore distinctive black clothing, sometimes with a grey overcoat.
The T's made their headquarters in the derelict Gotham Subway. However, since the place was destroyed by the Jokerz using a hijacked military vehicle, they may have since found another place to inhabit.