Slitherlink came from GAMES Magazine.
There are boxes with numbers; the number in a square indicates how many sides of the square have lines along the edge.
Lines are drawn dot-to-dot horizontally and vertically. Never diagonally.
Some dots may not be used. If a dot is used, then it will have no more or less than two lines radiating from it.
The object is to make one continuous loop within the box, with all boxes connected to the loop, no loose ends, fragments, or secondary loops.

This program was hacked together in a day by ripping the guts out of my paint by numbers program and coding in some new bits. There my be problems, though I believe I have squashed the worst of the bugs.
There may also be some confusing variables or routines. That's why.

I have provided a few puzzles, including a sample with a solution.

Left clicking draws a line between two dots. You have to be pretty close to the line, or you can drag over it.
Right clicking on a line will remove it. Right clicking on a blank line will X it out so that no line can be drawn. Right click it again to remove the X.

<CTRL Z> is the undo shortcut (there is also an undo in one of the menus)
you can undo up to 256 moves, though if you need that many, it probably won't help.

To make your own puzzles, use one of the existing ones as a template.
the numbers 0 - 3 appear on the grid, 4 is a blank square.
Progress files are automatically generated when you save progress

I have not worked on a routine to check whether the puzzle is solved. Someday.