What's a TTY? What's a TDD? What's a relay system?
A TTY is basically the same thing as a TDD. The phrase TTY (or Teletype device) is how the deaf community used to refer to the extremely large machines they used to type messages back and forth over the phone lines. A TDD operates in a similar way, but is a much smaller desktop machine. Since the deaf community has used the phrase "TTY" for so many years, it is still commonly used interchangeably with "TDD."
A relay system makes it possible for deaf people who use TTYs to call other people who may not have a TTY. For example, let's say a deaf woman wants to call her doctor. Deaf people use TTYs, a device that allows them to type their messages instead of speaking. Some businesses sometimes have TTYs and they advertise their separate TTY number for deaf people to use. But what happens if the doctor doesn't have a TTY? If this woman called directly to the doctor's office, they wouldn't be able to have much of a conversation because the doctor's receptionist wouldn't be able to understand the beeping sounds in her ear, and the deaf woman would just get garbled characters on her machine.
In the past, deaf people would either have to find an interpreter, a neighbor, or a relative to help them make phone calls, or they would just go into the doctor's office in person. A more efficient solution to the situation mentioned previously is the relay system. Now it's possible for the deaf woman to call the relay center in her state and reach a relay operator who answers the phone using a TTY. Through typing on the TTY, the deaf woman tells the relay operator the phone number of the doctor's office, and the relay operator dials the doctor's number using a second telephone. Once the receptionist answers, the deaf woman can then type a request for an appointment. The relay operator reads the typed message out loud to the receptionist. The relay operator would then "relay" the receptionist's message back to the deaf woman by typing it into the TTY. This goes back and forth until the conversation is finished.
[NOTE: The page was written 15 years after the DPN movement. Since that time relay services have evolved and video relay services (VRS) have become the most used relay services. This allows deaf people who use American Sign Language to communicate in the language they are most comfortable using. -updated 2010]