Technically, 4 conductors must be ran to the outbuilding:
2 hots
1 neutral
1 ground
The panel in the outbuilding is considered a sub-panel. Therefore, the neutral and ground should NOT be bonded in the outbuilding panel. Both hots, neutral and ground should be ran from the main panel to the outbuilding panel. In this case, no grounding rod at the outbuilding is required. Grounding is accomplished via the ground rod back at the main building/panel.
UNLESS...
There are no metallic connections between the main building and outbuilding (with the exception of the conductors for power). In this case, only 2 hots and a neutral are required to be ran from the main building to the out building. The neutral and ground SHOULD be bonded at the panel in the outbuilding. A ground rod must be installed at the out building and grounded to the panel. However, if you run say a telephone wire, coax (TV), water pipe or any other metal (conducting) material that connects the main building with the out building, the separate ground wire must be ran as explained above (and bonding screw removed).
Even if you have no metallic connections between the two buildings, the 4 conductor scenario is still valid and legal. When you consider digging a trench, buying and installing conduit, it makes sense to just run the extra wire and be done with it.
As others have said, do account for voltage drop and size your conductors accordingly. You only want to do this once.
