FM stands for frequency modulation. It conveys intelligence by varying the frequency of the radio wave. The FM broadcasting system we use today was developed by Maj. Armstrong back in the 'thirties. FM radio remains an inexpensive and popular source of music. (Maj. Armstrong also invented the regenerative and superheterodyne radio receivers, as well as numerous other devices. The superheterodyne circuit remains common to almost all radio receivers to this day, and Maj. Armstrong's contributions of the superhet and FM radio give him an equally valid claim to Nikola Tesla's pop culture status as The Genius Who Invented Radio But Got Screwed By the Man.)
FM radio broadcasting occupies the VHF portion of the radio-frequency spectrum (specifically, from 88 to 108 MHz). Signals in this part of the spectrum are not bounced off the F layer of the ionosphere. They can occasionally be bounced off the E layer, and in addition bending of the waves takes place almost every summer morining in the troposphere. In the early days of FM radio, from the late 'thirties on through the 'seventies, DXing was possible, but in the 'eighties the FCC allowed FM broadcasters to greatly increase the power they ran and shoehorned many new stations into the FM band, with the result that weak signal work is now very difficult in metropolitan areas (where almost all Americans live), because strong local stations occupy the entire band. My own best record in the old days was receiving a classical music station out of Washington, D.C. one summer day at noon during my lunch break. I was working on a loading dock in Houston, Texas at the time (1978) and had my 1969 Chevy pick-up parked in the middle of the open area where the trucks turned around. The previous owner had installed a Pioneer AM/FM stereo, which was the first radio I ever owned which didn't work worth a damn on AM, but obviously had good performance on FM. Given the time of day, I think this particular DX arrived via the E-layer, but it could have been tropo. One way to tell the difference between the two is the duration of the opening, but I had to get back to work!
In addition to FM broadcasting, frequency modulation is used by a wide variety of two-way mobile and fixed radio services, such as police and fire departments, delivery services, and baby monitors. It is also used by hams on the vhf bands.
There is also an active pirate FM radio movement in America today. The radio4all people are some of the most iconoclastic.