There are versions of Xbox and Playstation that are already loaded with Rock Band songs, while there are version of Wii which requires songs to be downloaded on the disk.
Rock Band for the Xbox 360 and PlayStation 2 and 3 shipped with 58 songs on disk, while the Wii version, released at a later date, contained 5 additional songs that were released as downloadable content for the game. For the PlayStation 2 and Wii versions of Rock Band, Harmonix has created Rock Band Track Packs that contain a selection of the downloadable content already offered. Rock Band Track Packs (called Song Packs in Europe and Australia) are a series of supplemental media discs for the Rock Band series of music video games. 
