The sawyer is faced with several different options for way log is to be cut. Depending on species of log and desired use for lumber that get milled off. Two main choices are flat sawn and quarter sawn.
Flat sawn lumber can be identified by arcing end grain and open, flame look on face of board. Flat sawn lumber is most economical to produce and there for is most common.
Quarter sawn lumber is almost exclusively used for wood working. With quarter sawn lumber a log is milled into quarters, quarters are then sawn perpendicular to grain. This gives lumber a unique characteristic with end grain being vertical. The face of boards show growth rings as alternating light and dark grain lines.
The big advantage to quarter sawn lumber is it's stability. Although it still moves it is a lot less prone to warping then flat sawn lumber because grain isn't subject to radial forces. Depending on what you are building you have choice of what type of lumber you want to use.
Soft wood is common and is easy to find throughout North America. Almost every lumber retailer carries plenty of softwood in stock. When buying DO NOT let attendant in yard select which lumber you are going to buy. Go look through lift and select which pieces you want because there tend to be more pieces in lift that are rejects for wood working.
It is a good idea to restack all boards that you rejected while looking through stack. This keeps rejects from warping and twisting beyond being useful and will keep lumber attendant happy.
Keep your eye out for defects when buying wood. this means looking at all four side of piece of lumber before selecting it. A board that looks good on one side may have wain on other side, something you want to avoid.
Hardwoods are a bit trickier to find at a regular lumber yard, especially out here in west. In British Columbia I can go to almost any private sawmill and find spruce, fir or cedar without a problem and pay a good price for it. Of course all these species are readily available, but try doing same for oak of maple. Good Luck.
I have been lucky to find an independant hardware store in Kamloops that supplies a wide variety of domestic and foreign hardwoods. They allow me look through all their stock to choose best pieces and even will cut a long board in half if that is all I need. Needless to say I value this relationship very high.