Adult Alternative is something of an all-inclusive term used to describe music played on radio stations with a music format which has also been called Triple A (or AAA, short for Album Adult Alternative).
The format is a broad mix of classic or heritage rock artists like Bob Dylan, Bruce Springsteen, and John Mellencamp, as well as what some refer to as Americana or modern country rock, as played by acts like Wilco and the Jayhawks.
The spectrum of Adult Alternative is a broad one, but the most common thread is probably that this is music enjoyed by adults who grew up on classic rock, but are still seeking out new sounds. Which is why bands as diverse as Radiohead, Dave Matthews Band, Ben Harper, and Coldplay enjoy airplay on these stations, while heavier bands like Metallica and Tool mostly do not.
Surprisingly, there are not all that many Adult Alternative Blogs out there, although Backstreets, a Bruce Springsteen news site updates on other AAA artists regularly. There are also tons of internet Adult Alternative radio stations on the blogosphere to listen to.