Corral is a puzzle originally from nikoli; their description is only in Japanese, so for external links you may have to settle for Wikipedia’s very brief article.* The object is to draw a single closed loop along the gridlines that does not intersect itself. The loop should enclose every number in the grid. Furthermore, if we treat the loop as a wall, the number tells how many grid squares in the loop can be seen from the number’s square when looking vertically or horizontally, where the number’s own square is counted. The solution is unique.
Below is an example puzzle and its only solution.
* Nikoli refers to this puzzle as “Bag” as the links show. Besides the fact that I find this name silly, I’ve always seen it called Corral, so that is what it will be here. I think this name is from whoever introduced the puzzle to the USPC.