Specifically, GooglePlay combined the content and infrastructure of Google Music, Google eBookstore, and the Android Market (which recently also started serving as an access point for video streaming. However, the integrated GooglePlay site is entirely cloud-based, so that all of the content is always available on any device that is Internet-connected. As the press release touted:
“Google Play is entirely cloud-based so all your music, movies, books and apps are stored online, always available to you, and you never have to worry about losing them or moving them again,”As with Apple's iCloud, a key feature of GooglePlay is the ability to not only have digital content available across your devices, but to automatically sync that content. Early reports suggest that in terms of cloud storage space - free "storage" for digital content acquired from them (they don't actually have to put whole files in a separate area for you - just keep track of what you've paid for and let you access their main storage to stream or download), a limited amount of "free" storage for other files you choose to put in the Cloud, and opportunity to rent larger storage lockers.
Sources - With GooglePlay, the Cloud Goes Prime Time, Wired.com Cloudline blog