TTY stands for Tele Type writer. TDD stands for Telecommunication Device for the Deaf.
Text Telephones are TTYs.
Ironically, the government has agreed that all TTYs were going to be called TDDs, but most Deaf individuals, active in the Deaf community will continue and prefer to use the term TTY when referring to their telecommunication device. TTY communication differ from normal telephone communication, but with a few pointers and a little practice, you will do just fine. Keep your sentences short and right to the point. If at all possible, have a printer to facilitate your life. Try to avoid asking too many questions at once.