Single sideband
Single sideband is a mode where the baseband (that is, the transmitted audio) is shifted in frequency (either above or below a center frequency) and transmitted. There are two sub-types of SSB modulation: LSB (lower sideband), where the baseband is shifted below the center frequency and inverted, and USB (upper sideband), where the baseband is shifted above the center frequency. By convention, LSB is used on and below the 40-meter band, and USB is used above the 40-meter band. For example, transmitting a 1000 Hz sine-wave tuning signal using USB (as is conventional) on a center frequency of 14.313 MHz would result in the transmitter transmitting a sine wave at 14.314000 MHz (i.e. 14.313 MHz + 1 kHz). Doing the same on 7.200 MHz but using LSB would result in transmitting a sine wave at 7.199000 MHz (i.e. 7.200 MHz - 1 kHz). An SSB signal has a bandwidth as wide as the audio being transmitted; if a voice signal is being transmitted, filtering usually makes SSB signals occupy 3 kHz of bandwidth.
An SSB signal is equivalent to an AM signal with the carrier (at the center frequency) and one sideband supressed, leaving either the lower or upper sideband to be transmitted. Most analog radios transmit SSB this way, first creating an AM signal at an intermediate frequency (IF) with a mixer and then using extremely narrow filters to filter out the unwanted carrier and sideband.
Since SSB is simple and spectrally efficient, it is the most commonly-used voice mode on HF.