Mounting a belated challenge to Apple’s dominance of the digital music retail market through iTunes, search giant Google unveiled its full music service, dubbed Google Music, at a press conference in Los Angeles on Wednesday.

Google launched a music cloud-storage service in invitation-only beta in May and has now opened that service up to all U.S. residents; users can store up to 20,000 songs in their personal cloud “locker” for free. More notably, the company announced that it would begin selling music through its Android Market, with all music purchases added immediately to the cloud and available on all connected Web and mobile devices.

“Other cloud music services think you have to pay to listen to music you own. We don’t,” said Jamie Rosenberg, Google’s director of digital content for Android, in a not-so-veiled dig at Apple, whose similar Match service charges a $25 annual fee.

Label groups participating in Google’s retail option include Universal Music Group, Sony Music Entertainment and EMI, as well as large indie groups such as Merlin, the Orchard and Beggars Group. Google has yet to reach an agreement with Warner Music Group, whose content will not be immediately available.

“We anticipate adding new partners as they choose to come onboard,” said Zahava Levine, Google’s director of content partnerships.

Google’s retail site, as demonstrated, is not notably different from Apple’s, with tracks ranging in price from 69¢ to $1.29. Site will offer a number of exclusive tracks from the likes of the Rolling Stones, Coldplay and Busta Rhymes (who was seated in the audience at the conference), as well as daily freebies.

Execs touted the ability to share purchased music with friends on the company’s Google+ social network platform. Upon buying a track or album from the Android Market, a user can send a streaming copy by using Google+ or entering individual email addresses, and recipients can listen to full recommended tracks and albums one time through for free.

T-Mobile subscribers will also have the option of adding Google Music purchases to their monthly phone bill.

Another significant distinction involves Google Music’s accessibility to artists who self-release music. While iTunes requires an intermediary (whether it be a record label or a digital distributor such as TuneCore) to place music in its marketplace, Google will allow bands to upload music directly for purchase, as well as to determine and change their own pricing.

While Google as a company certainly has the heft to challenge Apple, unseating iTunes will nevertheless be a tall order. ITunes has had just over a decade to establish itself as the world’s largest music retailer, and the distinctions that Google has drawn may simply be seen as slight improvements to an established model.

The absence of WMG content (which had a 20% market share in 2010) could also hurt, especially as that content is widely available on iTunes and Amazon and through on-demand streaming services.