A Text Telephone (TTY) is a device that allows individuals who are deaf, hard of hearing, or have speech disabilities to communicate over the telephone. It uses a keyboard and a display screen to send and receive text messages in real-time, enabling users to have conversations without relying on voice. TTY devices connect to standard telephone lines and can also communicate with other TTY users or relay services. Relay services involve a communication assistant who reads the typed messages aloud to the hearing party and types their spoken responses back to the TTY user, ensuring effective communication between different users.