Traditional Radio Broadcasting
AM - Amplitude Modulation - where the amplitude of a carrier wave is varied in accordance with some characteristic of the modulating signal. For more please see: The Analog AM/FM Radio Tuner Still Has Fans - Thousands
FM - Frequency Modulation - A method of impressing data onto an alternating-current (AC) wave by varying the instantaneous frequency of the wave. For more, see: History of AM and FM Radio
New Tech Broadcasting
HD Radio - HD Radio technology transmits digital audio and data alongside existing AM and FM analog signals. According to Ibiquity, the developer of this technology makes "...your AM sound like FM and FM sounds like CDs."
Ibiquity also says HD Radio offers "FM Multicasting - the ability to broadcast multiple program streams over a single FM frequency", "Static-free, crystal-clear reception", and "A variety of 'data services' including text-based information..." For more, see: HD Radio: What it is and What it is Not
Satellite Radio - SIRIUS XM Satellite Radio (now one company) delivers programming to millions of listeners who are willing to pay for special radio equipment along with monthly subscription fees.
Programming is beamed from earth to satellite, then sent back to earth. Special antennas receive the digital information either directly from the satellite or from repeater stations which fill in gaps. For more, see: Satellite Radio Merger Creates New Programming Options for Subscribers
Internet Radio - It feels like radio and sounds like radio but it's not really radio by definition. Internet Radio - also referred to as "streaming" provides the illusion of radio by separating audio into small packets of digital information, sending it to another location (your computer) and reassembling the packets into one continuous stream of audio. For more, see: 10 Significant Milestones in Modern Radio - Shoutcast 1998