Referenced in the film, and in record stores, as the Awesome Mix Vol. 1, the soundtrack is basically an epic mixtape of pop and soul classics from the '60s and '70s, including songs by Marvin Gaye, David Bowie, The Jackson 5, and The Runaways.
In the movie, the Awesome Mix is a cherished memento of intergalactic swashbuckler Peter Quill, handed down from his deceased mother. But apparently, Quill's fictional nostalgia is contagious, because the IRL version of the soundtrack just hit No. 1.