RTTY used a variety of different modulation methods, of which frequency shift keying was the most common. The coding used was the 5-bit ITA2 code (also known as the Baudot code), which was used asychronously with start and stop bits.
RTTY was not fast by modern standards; a typical baud rate for RTTY operation was 75 baud.
The combination of low baud rate with robust FSK modulation made RTTY highly resistant to most forms of radio interference, second only to Morse code.
RTTY is still in practical use today (2001): see the link to German weather broadcasts (RTTY at 50 baud using FSK) below. RTTY systems are also fielded by radio amateurs.