Luna was originaly used for Linden World, a theme park sponsored by Linden Lab for residents to build amusement rides in, along with Coney. When the Linden World idea was scrapped due to somewhat disinterest Luna was converted into a shopping sim.
Now a shopping sim owned by the Lindens, the shopping mall itself being called Luna Oaks. Short term (3 month?) leases are awarded via a lottery for stalls and small stores in the sim, which has generous prim usage and great FPS. Larger stores are auctioned via the Linden auction website. Luna Oaks is one of the few (if not the only) Linden owned and run shopping malls in SL. It features work by Xenon Linden and Ben Linden (scripting for the fountain).
The trolly that runs through the city sims also makes a stop in Luna.