T-Girl (short for Trans-Girl) is the generally accepted "polite" way of referring to Transgendered or Transsexual Girls. It is a general term that covers a wide range of people; Cross Dressers, Transvestites and Pre or Post Op Transsexuals may all be referred to as T-Girls without too much risk of offending anyone. Many people however, are not bothered about causing offense to other people, and may refer to such people in a derogatory fashion using words or phrases such as Shemale, Tranny, He-She or Chick with a Dick.
Most T-Girls would not be offended as being described being a T-Girl, although it should be pointed out that many Post-Op Transsexuals would probably prefer to simply be referred to as Girls.
Note: The terms Pre-OP and Post-Op refer to before or after a Transsexual may have Gender Reassignment Surgery.