How do Leaderboards work in Challenge Mode?

Leaderboards are integrated into the game through Challenge Mode -- the pure expression of Riptide GP: Renegade skill.

In Challenge Mode, you compete with your Friends for the best time on each track.  When you start a new race, the game downloads your Friends' best ghosts, and you race against them.  When you set a new personal best time, a recording of your "ghost" is uploaded to the Leaderboard for your Friends to race against.