Since I randomly talk about general woodworking things, I figured I'd explain what a board foot is. It's a term that is particularly important when buying lumber, designing projects, and pricing jobs.
When buying lumber, typically, you pay for it by the board foot. There are exceptions to this, which I'll list eventually, but most lumber is sold by the board foot.
Explanation after the jump.
Updated with wisdom from my dad.
A board foot is a measure of volume, not area. One board foot is 12" long x 12" wide x 1" thick, or 144 cubic inches. Another piece of wood measuring 6" x 12" x 2" is also a board foot.
Most boards you buy aren't 12" wide. So typically you take the length of the board in inches, multiplied by the width of the board in inches, multiplied by the thickness of the board in inches, and divided by 144 = board feet. ( (l" x w" x t")/144 = board feet ).
If you do the math, you realize that if you're buying 8/4 stock, then you get shorter and narrower boards per board foot, which causes the price of your lumber to go up quickly. The same 4" wide 8 footer in 8/4 is twice as many board feet as is the 4/4 board (5.3 bf compared to 2.6 bf) . Most lumberyards also increase the price of 8/4 stock so it isn't the equivalent of buying 4/4, it's more expensive per board foot.
There are some things that you buy by the lineal foot, not the board foot. If you buy trim or molding, it's usually priced per lineal foot.
Certain thin stock is priced per square foot, and if you have the ability to resaw (to put wood on edge and cut it into two thinner pieces), then it's often a better deal to buy the 4/4 and to resaw it to thin stock. For example, this is the case for bloodwood. You spend almost 5 dollars more per board foot of 4/4 than you do per square foot of 3/8" thick stock. You're better off to buy the 4/4, resaw it in half so you have two pieces that are each about 1/2" thick, and then clean it up on the planer to make your 3/8" material.
Also, plywood isn't sold per board foot, it's usually sold in 4' x 8' sheets and priced by thickness. Some things (baltic birch plywood comes to mind) can be sold in 12" x" 30" sheets or 16" x 30" sheets or 5' x 5' sheets. I'm not really sure why on that front. Home Depot sends half and quarter sheets of some plywood or MDF (medium density fiberboard); it's almost always cheaper to buy a full sheet and cut it down to size yourself.
The more you know.
Update: My dad rightly points out that I didn't mention that this post describes buying lumber from a lumber yard in rough form. If you go to Home Depot and buy a board of maple hardwood, you should expect it to be planed to 3/4" and to be priced by the lineal foot. It is not a cost efficient way of buying lumber, but if you don't have a jointer or a planer to clean up the rough cut stock, then you're kind of stuck paying their upcharged rates.