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HF
Also from: High Frequency
HF radio frequencies are between 3 and 30 MHz, although transceivers are usually from 2 to 30 MHz.
HF radio waves are propagated simultaneously by ground, direct and sky waves. Ground waves can travel about 100 km over land, and about 300 km over sea. Direct waves are line of sight. Sky waves are transmitted by skywave propagation. This is where the electromagnetic waves at HF frequencies are refracted back to the earth's surface by the ionosphere.
Due to properties of the ionosphere, the general rule on the use of frequencies is the higher the sun, the higher the frequency. This also applies to the distance the signal needs to travel: the greater the distance, the higher the frequency.
SSB and AM
SSB (or single sideband) is used to compress the transmitted information (voice) into a more compact signal. This is done to reduce the power required to transmit the signal over a given distance, and reduces the bandwidth of the signal (compared to AM). SSB also allows for more channels due to the use of only one sideband.
AM signals are double sideband.