By default, the Recovery partition is not mounted (ie. it is not visible to the operating system), but mounting it is an easy job: -
1 - From Finder, go to your Applications > Utilities folder
2 - Launch Terminal.app
3 - Run this command to mount the Recovery HD partition: -
diskutil mount Recovery\ HD
4 - You're done! BaseSystem.dmg contains the (very) cut-down version of MacOS that is used to connect to the internet and download the full Lion install.