I've referenced this before, and the term is obscure enough that it deserves a decent explanation instead of my usual sarcasm-laden posts from this category.
AOR stands for album-oriented rock, and was, essentially, the first major type of alternative radio. The format got its start when FM stations were prohibited from echoing the playlists of their AM counterparts, who predominately played major singles, in the late-60s. Fortunately, the mid-60s had ushered in an era of album-based experimentation, marking the beginning of the LP as the main form of artist expression. This, in turn, had its origins in the late-50s concept albums put out by Frank Sinatra (which included major critical successes like No One Cares, Come Fly With Me, and Sinatra's personal favorite of his discography, Only The Lonely).
The LP-as-artistic-statement got a further boost from the Beatles' post-Rubber Soul output, especially the landmark Sgt. Pepper album. Suddenly flush with songs, FM DJs began playing songs that hadn't been selected as singles, giving them a much wider range of material to work with.
The end result was that a certain branch of music was suddenly given a great deal of widely played material, especially during the 70s. One key example is the Eagles; many of their best known songs (such as "Desperado") were never issued as singles, meaning the distribution of such tracks was due to the AOR format.
The format is extraordinarily wide-reaching (its influence can be seen in college radio and most alternative stations, who play bands that probably never had singles in the first place) but the stations that fell directly into the form disappeared in the late-80s and early-90s... to be replaced by classic rock stations. Which is why the term AOR tends to crop up so much.