Grain refers to the direction of the fibers in a piece of wood.  In side grain or edge grain cutting boards, the fibers run parallel or at 90 degrees to the cutting surface.    When using these types of boards the fibers are cut during use. 

 

Consider and exaggerated example.  Lay a paint brush on the table and run a knife across the bristles.  Some of the bristles are cut and are unrepairable. 

 

These cuts add up over time.  Food and liquid particles become logded in these cuts and can harbor bacterial growth.  Most woods have antibacterial properties but there are limits to what they can do. 

 

In end grain cutting boards, the grain runs up and down.  From the underside of the board to the top side.  When cutting on these boards, the wood fibers move apart when the knife blade is drawn across them, and then close up when the knife is withdrawn, squeezing out foreign material, making them much more sanitary and easy to clean.  A knife will keep its razor sharp edge much longer and will leave only a faint line where the cut was made.  

 

Using our paint brush example again, holdng the brush with the bristles facing upwards,  we run a knife blade through the bristles.  They part to allow the blade to pass through and then return to their original position once the knife is removed.  

 

Because the fibers are not being cut to the extent that they are in other types of boards, the knife stays sharper, the board lasts much longer, it is easier to clean, and wood particles are not getting into your food.  

 

The disadvantages of end grain cutting boards are that they need to be thicker to prevent warping, and they are more time consuming to make, hence more expensive to buy.  With a little care and maintenance however, they have the potential to last generations.