Radioteletype (RTTY) is one of the oldest data modes still in everyday use: it began with mechanical teleprinters and now runs on a computer. It sends text by shifting between two frequencies. It is simple, needs little bandwidth and is a staple of HF contests.
Mark and space
Looks tuned, prints garbage? Check sideband, mark/space and baud rate.
A transmission is a stream of two tones. The two frequencies are called mark and space, and they are 170 Hz apart in the usual amateur setup. The receiver compares which tone is stronger at each instant and turns that into 1s and 0s. Comparing two tones is simple, which is why RTTY has run on everything from machinery to laptops.
Baudot
Baudot: fixed 5 bits plus framing. Varicode: short codes for common characters, so capitals are slower.
Each character is sent as 5 bits (Baudot code) with a start and a stop bit around it. Five bits only gives 32 values, so two special shift codes switch between letters and figures. If a shift code is lost to noise, the rest of the line can print as figures instead of letters until the next shift, which is why RTTY gets a familiar scrambled look when it fades. There is no error correction.
Making the tones
FSK = the signal jumps between set frequencies (two, for RTTY). Direct FSK moves the VFO itself.
Direct FSK shifts the transmitter's own frequency. Audio FSK (AFSK) feeds two audio tones into an SSB transmitter, which is how most people do it with a computer and sound card. With AFSK on an SSB radio, the sideband decides which tone ends up higher, so convention says to use lower sideband.
In practice
RTTY contests are fast and noisy, using repeated short exchanges. For casual conversation it is easy to set up and works from simple equipment, though newer modes outperform it on weak, fading signals.