CW (Continuous Wave)
This uses the narrowest signal possible. Morse code is used to communicate with CW. Due to the narrow bandwidth the transmitted power is concentrated over the smallest segment and tends to work when no other mode will.
AM (Amplitude Modulation)
This mode consists of a carrier at the center of the transmission with two side bands on each side of the carrier. The Amplitude of the sidebands are varied to carry the signal.
SSB (Single Sideband)
This mode is related to an AM signal. The carrier and one of the sidebands are suppressed leaving the transmitted power concentrated over just one sideband. This mode is 3KHz wide. LSB (Lower Sideband) is typically used below 10MHz and USB (Upper Sideband) is typically Used above 10MHz. The 60 Meter band is a notable exception to this. On the 5 channels allocated to secondary amateur use only USB can be used for phone.
FM (Frequency Modulation)
This mode has a carrier that is modulated by varying the frequency to carry the signal.