R268 Basically, AM radio was "Top 40". They played whatever was a hit. Mainstream pop, R&B, country, hard rock, adult contemporary, novelty records, bubblegum...if it was a hit, it got played somewhere on AM. They also almost exclusively played songs that were released as singles. The DJ's were high energy personalities.
FM was supposedly less crassly commercial, and less hit driven. They mainly played artists that had a modicum of critical respect, and an image as a "serious artist". They often played album cuts that were never released as singles, or that were released as singles, but weren't successful. They concentrated on southern rock (or country rock), singer/songwriters, hard rock, and prog rock. If a song was released in an edited version as a single, they would always play the long "album version"...and sometimes those were really, really long.
For all of its supposed seriousness and progressiveness, FM was, in practice, actually very restrictive and limited in a lot of ways. There was a serious dearth of black artists on their playlists, with the exceptions of Jimi Hendrix and Stevie Wonder. This was probably partly due to racism, and partly due to the perception of R&B as "too commercial"...you know, a bunch of guys or girls all dressed alike, doing choreography. Yet they the had no problem playing the hell out of the Average White Band and Boz Scaggs. So white artists playing R&B got played, while black artists mostly didn't (Stevie being the one exception). Even bands like the Isley Brothers, who did serious "message" songs, were left out. FM was also very late to embrace the punk and New Wave, clinging stubbornly to the same old sounds while this musical revolution was happening right under their noses.
A lot of times, it was more about an artist's image than their actual music when it came to who got played on FM. Musically, I don't think there's a ton of daylight between somebody like James Taylor, who was an FM (and AM) fave, and John Denver, who only got played on AM. JT leaned a little bit more into R&B I guess, while JD leaned more toward straight country, but basically they weren't all that different. They were both white guys with acoustic guitars who wrote and sang their own country-tinged, folkie-pop songs. But JT was cool, sullen, and a serious drug addict. JD was the epitome of uncool...a total dork, who had no problem clowning it up in cornball TV variety show skits next to "establishment" entertainment figures. So, JD couldn't get arrested on FM, even though something like "Rocky Mountain High" would have fit very nicely into the FM format. Melissa Manchester did get played on FM in the beginning, her first few albums were very much in the early 70's singer/songwriter vein. But when she started recording overly commercial schlock written by others, like "Don't Cry Out Loud", she was no longer seen as "serious" and was exiled to AM only. Same thing with Neil Diamond. He was cool enough for FM until about the time of the Streisand duet, then they dropped him.
A lot of artists got played on both formats. They would have big hit singles that got played on both, while FM would dive deep into their album cuts: The Rolling Stones, all of the ex-Beatles, Chicago, Jim Croce, Linda Ronstadt, Elton John, Eric Clapton, Rod Stewart, Paul Simon, Billy Joel, Peter Frampton, Fleetwood Mac, and so on. Others, like Jethro Tull, Pink Floyd, Deep Purple, and Led Zeppelin, were mostly FM, but were able to come up with the occasional AM hit single. That didn't work in reverse, though...an artist perceived as AM wouldn't get played on FM under any circumstances. They were never going to play The Carpenters, no matter how many Klaatu or Leon Russell songs they covered. Neil Sedaka wasn't getting FM play, no matter how much he recorded and palled around with Elton John. Likewise Helen Reddy, Tony Orlando and Dawn, Cher, ABBA, and Barry Manilow.