Wood Cutting Boards End Versus Edge Grain
End-grain cutting boards, at left, show the tree-ring cross section on the work surface, while edge-grain boards (right) show the length of the wood fibers.
A tree trunk grows vertically, and its fibers run along that length, providing channels through which water and nutrients flow from the roots to the branches and leaves. There are a lot of ways to cut up a tree trunk, but for the sake of clarity I'll keep this discussion simple.
When you crosscut the trunk, you expose what's called the end-grain. That's the cross-section of fibers that we see as the tree's concentric rings. Cutting boards for which the end grain is on the cutting surface—that is, where the tree's rings are visible on the surface—are often referred to as butcher blocks.
When you cut the trunk vertically, on the other hand, you expose a side view of the fibers running lengthwise; this is often called the edge-grain. (As I said, there's more to it than that, but for the purposes of cutting boards, I'm limiting it to these two terms.)
Think of it like a bunch of plastic straws all glued together: They're just like the fibers running up and down the length of a tree trunk. If you were to cut them into cross-sections, you'd get rounds full of visible holes. Make these hole-filled cross-sections the surface of your cutting board, and it'd be an end-grain board. If you were to cut the bundle of straws lengthwise, you'd split the straws into smaller groups of straws that are all the same original length; glue those smaller straw bundles together side-by-side so that they fashion the work surface of a board and it'd be an edge-grain board.
The edge-grain board (top) has end-grain sides, while the end-grain board (bottom) has edge-grain sides.
Note that an end-grain board will have the edge grain visible on its edges, and an edge-grain board will have the end-grain visible on its edges.
Which is better? Well, that's hard to answer. End-grain boards, where the tree rings are visible on the work surface, are ever so slightly more gentle on your knives, since the blade can slip between the exposed individual fibers. You can't see this because the fibers are practically microscopic, but if you could zoom in, it'd look like a knife sliding between brush bristles, which close back up as soon as they knife is lifted away. This does less damage to the blade over time. The board holds up better, too, because the fibers can reset after the knife is pulled away; an end-grain board can still develop scratches, but they won't be as severe as on an edge-grain board under the same conditions.
On an edge-grain board, the knife comes down sideways onto the tree's fibers, splitting them like broken strands of spaghetti. This will wear down the blade somewhat faster, and it'll lead to gashes in the wood that won't heal so easily. The board will eventually develop deeper scratches and can even splinter, though it'd take a lot of abuse to get there.
Sounds like an end-grain board is better then, right? Not so fast. End-grain boards are more difficult to make, which means they're more expensive than an edge-grain board of otherwise similar build and material quality. They also have a lot more glued seams compared to the long strips of wood that make up an edge-grain board. Seams are frequent points of failure, which puts end-grain boards at higher risk (assuming equal construction and material quality).
End-grain boards, like the one at left, absorb liquids more rapidly than edge-grain ones do. That's why it's even more critical to keep an end-grain board well oiled, to prevent it from drinking every drop of water that touches it, and warping as a result.
End-grain boards are also more prone to warping and cracking, since all those exposed fibers absorb and release moisture much more rapidly (one of their purposes in the tree was to transport water, after all). An end-grain board will expand and contract with the weather and seasons, and will be harmed more dramatically and rapidly from over-exposure to water than an edge-grain board will.
That said, a well-made end-grain board will still have a better chance at survival than a poorly made edge-grain one. This is a key point—there's no type of wooden cutting board that is guaranteed not to fail. High-quality expensive ones may be less likely to crap out on you, but some percentage will crap out nevertheless, even if you care for them properly (here's how to treat a wooden cutting board right).
The type of board you choose should depend on a few factors. First is price. If you can't afford a good end-grain cutting board, a good edge-grain will be your default. Second is how heavily you plan to use the board. Cook a ton? You may see a small benefit from the blade-protecting qualities of an end-grain board, though the difference is hardly dramatic. Third is care, and you have to be honest with yourself. Are you really going to saturate your cutting board regularly with food-grade mineral oil to keep it quenched and less susceptible to warping, cracking, and water damage? Or are you going to neglect it the same way you've neglected those fancy knives you should have resharpened four years ago? If you're ready to put in the care, consider an end-grain board; if not, go for an edge-grain, which will generally tolerate abuse better.