Google Music which was recently launched is a web music player where you can add your own song collection from the desktop manager. But when you want to play the songs, you need to open Google Music from the web browser or your phone. Google Music Player is a desktop player for Google Music which lets you play songs directly from its interface. It also has many additional features like minimized player, system tray management etc.
The player is a web version of the music service inside an app, you need to first login to your Google Account and then access the Google Music. When you minimize the player, you can play/pause buttons along with next and previous keys.
Please note, you need to have an invite to access Google Music (if you don’t have an account) and the service is currently US only.