With the SingSong trial you can only karaoke various songs by Creative Commons artists. The full version allows you to karaoke any song in the world. To karaoke a song, all you need is a "note file" and digital music file (MP3, OGG, etc). SingSong comes with note files for hundreds of popular karaoke songs. To karaoke one of these you only need to provide the music file. SingSong does not come with the music files.
Below is a full list of note files that come with SingSong. For any other songs you want to karaoke, the Note Editor can be used to make the note files needed. You may also visit the forums to request note files or post note files you have made.