If you've ever done drywall work, you know the corners are the really tricky part. These no-coat drywall corners are ready-made of high-impact plastic backed with joint tape for an easy, tight bond with mud. Shove the corner into a bed of mud-no need for screws or nails. Feather the edges with compound, and you're done.
For more on drywall, consider:
Drywall vs. Blueboard
Quick Tip: Taping Drywall Joints
How To: Finish Seamless Drywall