Rules
Draw a single closed loop along cells edges, such that the loop does not cross itself and the clues in cells indicate how many of its four sides are segments of the loop.
Example
Here is a grid of Slitherlink and its solution.
Solve the example online here.
History of Slitherlink
According to the WPC unofficial wiki, Slitherlink is a genre first published by Nikoli in 1989. Its rules were created by 矢田レーニン (“Yada Renin”).