Any Vehicle that is already in the roundabout has the right-of-way, as opposed to vehicles that are approaching the roundabout.
Traffic approaching the roundabout from any direction will enter the roundabout in a yield condition. This means that when arriving at the yield sign, the driver yields the right-of-way to any vehicles already in the roundabout.
When it is safe to enter the roundabout, vehicles proceed to the right (counter-clockwise) and exit at the appropriate street, If there are no vehicles in the roundabout, or if there are vehicles on the other side that provide a sufficient gap to enter, drivers may enter without stopping.
Even if heading to the left, you still enter the roundabout to the right and follow it around the center island until you reach your exit point. Under no circumstances do you attempt to go left.
This speed is beneficial in that you will be interacting with bicyclists (who will be traveling at slower speeds in the roundabout) and with pedestrians.
An informational flyer on roundabouts is provided here.