Horns sets are half-court offensive systems that are used at virtually every level of basketball. It involves 2 players (usually the 4 and 5) positioned on the elbow of the key, 2 players in the corners and a ball handler at the top of the key.
In the most traditional Horns action, the 4 and 5 are the screeners, and the ball-handler uses the 5 or 4 screen. The screener will roll to the basket, with the opposite big lifting for potential high low action. The 2 and 3, stay in the deep corners as shooters.
Horns actions have evolved into some sophisticated offensive sets that include both dribble and pass entries.
The team at www.basketballimmersion.com have put together a video that provides a variety of examples of various horns sets.