As has been mentioned previously mentioned the carrier is turned on and off to represent the dots and dashes, commonly known as dits and dah's, of the Morse code. While telegraphy is no longer routinely used in military and maritime services it is still very common in the amateur service, particularly on HF, and is likely to remain common even after the international requirement for morse proficiency for all HF licencees is dropped probably in 2003.
Most phone (voice) activity on HF is SSB. An AM signal consists of a carrier and two copies of the voice information that modulate it's amplitude, called sidebands. The modulating signal affects both the top and bottom half of the carrier identically. It is more efficient if we filter out the residual carrier since it contains no actual voice information, and then filter out one of the sidebands. On frequencies above 14MHz the lower sideband is filtered out and the upper side band (USB) used. Below 10MHz, the lower sideband (LSB) is used.
SSB, correctly called suppressed carrier single side band, uses less bandwidth than AM, and because it is also much more power efficient is the primary mode used for long distance voice contact.
Tuning at the receiver must be very accurate since the receiver must effectively reinsert the carrier before decoding the original modulating signal. Even a small tuning frequency error will render an SSB signal unintelligible.
This mode of communication is commonly used for transmission of computer
data. It consists of two tones used to represent the 1's and 0's of digital
data. It can be used for radio-teletype (RTTY) where basic text is sent via
radio, as well as for the sending of more complex data. In some instances more
complex data including full colour pictures may be sent using this mode or
derivatives of it.