It is supposed to contain the entire digital catalogs of participating labels, which include all but one of the major record companies. Beyond that the vision is unclear. There will be some free, ad-supported streaming of music, which sounds awful; some downloadable music and maybe a subscription option. As the Times pointed out, there is a big reason to doubt whether MySpace can pull this music thing off:

MySpace will have to prove that it can actually sell music. Though the company earns $70 million a month in advertising for the News Corporation, according to estimates by Pali Capital, it has never successfully sold products on a wide scale. A download service for independent music, begun in 2006 with Snocap, a music start-up, was considered a disappointment.

[Times]