Whenever two pieces of wood come together they form a joint. There are several basic wood joints that can be used, though, and some of them have further variation based on where they're placed. Each basic joint has its own uses, strengths and weaknesses, and understanding each type helps you to choose the joint that is best for your project.
A butt joint is a very simple corner joint in which two straight pieces of wood are held together either by nails or screws, or with dowels. In a dowelled butt joint the dowels protrude from one of the two wooden pieces and are fitted into holes in the other pieces of wood. Glue is often used to strengthen the joint. The same is accomplished by nailing, or using screws to penetrate both pieces of wood. These joints are quite strong and used often in manufacturing. The joint creates an L-shaped corner.
You make a dado joint by cutting a niche in one piece of wood that is big enough for a second piece of wood to slot into. This type of joint might be used in a bookcase or other shelving, where the supporting sides are cut to allow a shelf to slide into place. Glue, screws or nails can all be used to reinforce the joint.
A rebate joint is very like a dado joint, except that you cut the niche at the end of one piece of wood and the other sits down into it. For example, the top shelf of a bookcase or a table could have a rebate joint. In that case the top shelf or table top would sit on a "ledge" created by cutting out a piece of the supporting wooden structure.
A lap joint is a joint where two pieces of wood have been notched to fit together. Such a joint is very strong, and is made by sawing halfway through both pieces of wood and then fitting the notches into one another. It can be used as an end joint, to form a corner, in a supporting structure so that two pieces of wood can intersect without weakening the structure.
These joints are seen often on drawers and cabinetry. A notch is cut in one piece of wood and a spur cut in the other. The spur fits into the notch and the two pieces are held together. It doesn't have to be a single spur and a single notch; in fact more often there are a series of both, interlocking something like a zipper to hold together tightly.
Mortise and Tenon
This joint is formed when a long spur is cut on the end of one piece of wood and the other piece of wood is cut to fit around that spur. It may be like a whole, that fits onto the spur and surrounds it entirely, or it may only surround three sides of the spur. This forms a strong joint, especially when reinforced with screws, glue or nails.