The Rules
The goal of Slitherlink is to draw a single loop on a grid that does not branch or cross itself.

Draw the loop by setting lines on the grid.

Some cells in the grid have numbers inside them.
The number represents how many of its sides are segments in the loop.

How it works
Action To
tap set a line
double tap cross out a line
long tap set a bookmark
long tap bookmark reverse to bookmark
icon back undo last action
icon forward redo action
Solving Methods

You can find a good article about Slitherlink solving methods on Wikipedia.