The different sides of a piece of wood are labeled as either end grain or edge grain. Wood cutting boards are primarily made using the long or edge grain of a board because they are cheaper to manufacture, using less wood and less materials to build. End grain cutting boards are much more labor intensive to make but are the kindest to your knives.  An end grain cutting board will show fewer knife marks and last significantly longer than an edge grain cutting board.
When you cut on a edge grain cutting board you are cutting along the long fibers of the wood board (imagine dry spaghetti laying on its side). As you cut, the wood fibers are less flexible, and ultimately break, leaving knife marks in your board. With end grain, the wood fibers are standing upright (again, imagine a handful of dry spaghetti standing upright). As you cut, the wood fibers separate, allowing the knife blade to pass through them. When the knife is removed, the wood fibers come back together and mend themselves. As a result, end grain is significantly easier on your knives and results in fewer knife grooves on your board which means your board will look newer longer.
While edge grain wood cutting boards can be sanded and refinished, the wood fibers are broken which requires more sanding to remove all of the grooves. Since end grain boards are self-healing, they will last longer and refinishing can restore their original beauty.