Numberlink is a puzzle originally from nikoli; you can see their description of the puzzle here. The object is to connect each pair of numbers with a path passing through adjacent cells. No square may be used twice. Additionally all puzzles on this blog will have their solutions use every cell in the grid, but this constraint will never be needed to make the solution unique.
Below is an example puzzle and its only solution.
(This type of puzzle requires an approach that is a bit different from other puzzles on this blog. For some tips for solving them, you may want to refer to this post of mine.)