Unlike cloud music services like Spotify and Grooveshark, Google Music allows you to upload tracks from your own music library to the Google Music servers. Your music from that moment is accessible anywhere with an Internet connection.
Google allows you to add 20,000 of your own songs to your library. This is around 80GB of music if the average track is 4MB. While the service is in beta it will be free, prices for the service after beta have not yet surfaced.
As far as I can tell Google Music does not compress your music, unless it is FLAC, and supports a wide range of audio formats. A full list of acceptable file formats can be found here.
Google Music is available on multiple platforms. You can listen on the move with the official app for Android and listen on your iPhone with the very impressive mobile web app. PC’s and laptops can access the service through a web browser.
If you run a normal broadband connection chances are your upload speeds are quite poor. It can take a long time to upload your albums to Google Music. I have around 60GB of music in my iTunes library which unfortauntely wont be going to the cloud anytime soon.
Droptunes - Droptunes is very similar to Google Music and allows you to play music stored on your Dropbox account.
Spotify - Both free (limited) and paid services Spotify is leading the way in streaming music services.
Grooveshark - Free but less organized than Spotify.