Google Play is a digital content service from Google which includes an online store for music, movies, books, and Android apps and games, as well as a cloud media player that supports uploading a user's own music and buying music at Google Play Store.
Source: Google Play on Wikipedia, Official website
Google Play Music is currently available in the following countries: Australia, Austria, Belgium, France, Germany, Ireland, Italy, Luxembourg, New Zealand, Portugal, Spain, United Kingdom, and United States.
Google Play Music All Access is currently available in Australia, Austria, Belgium, France, Ireland, Italy, Luxembourg, New Zealand, Portugal, Spain, United Kingdom, and United States.
You can find up-to-date country availability details on Google Play support page.
Because of a bug in GStreamer 1.0.x playback is stuck on song loading on newer systems (Ubuntu 13.04, Fedora 19, Arch Linux). Nuvola Player 2.1.x contains a workaround for the issue: Go to menu Application → Preferences → tab Service and check "Disable HTML5 Audio support when service is loaded."
Since a maintainer of Google Play Music integration is living in a country, where Google Play Music is not officially available, he doesn't have access to the store and cannot ensure its proper functionality. All store links are opened in a default web browser for this reason. (Bug #1175454)
Google Play Music is able to play music via both the HTML5 audio and the Flash plugin. Flash plugin is used by default, but you can enable HTML5 Audio in Music labs: Cog button → Music labs → HTML5 Audio. However, Google Play Music All Access & Radio requires Flash plugin, HTML5 Audio is not supported for this feature.