Slitherlink is a logic puzzle where the goal is to draw a loop so each number has that many edges of the loop adjacent to it.
Slitherlink is a combinatorial logic puzzle usually played on a square grid.
The rules are simple. Some of the squares contain a number. The objective is to mark the edges on the grid in order to form a single closed circuit with no intersections.
The number of edges of every square that contains a number must match with that number.
Variants exist with other grid shapes, for example see Loopy.