Oversteer is when the rear wheels are carving a larger arc than the front wheels or the intended line of the turn. Rear "slip angles" exceed those of the front tires. This is often described as a "loose" condition, as the car feels like it may swap ends, or be "twitchy."
This condition can be caused by "power oversteer", where you need to reduce power in order to bring the back end back into line.