Plain sawn (or flat-sawn) lumber has the growth rings of the tree parallel to the board’s broad face. You can usually visualize this by looking at the end grain of the board. Plain sawn wood highlights the grain, loops and growth swirls of the wood and typically features a “cathedral” grain pattern. This is how the vast majority of lumber is cut.
Check out this video for a quick rundown of lumber terminology – Video: A Lumbering Feeling