By Ronan 
The jointer is a high-speed, stationary power tool. It has a table consisting of two adjustable surfaces: an infeed and an outfeed. Between these two horizontal surfaces (under a safety guard) is an opening below: razor-sharp knives that rotate at high speed. By adjusting the height of the infeed surface, the blades will shave wood away from a board fed through the jointer.
An adjustable fence lies at right angles to the table surface. This allows the adjustment of the table’s width to the width of a board being shaved by the blades. The best best 6-inch jointers can only plane board widths up to 6 inches. An 8-inch Jointer can plane up to 8 inches, etc.
Why the need for a jointer?
If you were to look at a piece of fine furniture made out of individual pieces of wood, you would probably see a board on the left side that is the same size as the corresponding board on the right side.
Why is New Lumber Warped?
Kiln drying and aging of lumber can cause the edges to become malformed. The board might be crooked or twisted or both. And when these boards are laid side by side, they can have gaps.
Gaps might not be a problem in building homes or in some woodworking projects, but in trying to produce two or more boards of equal widths with perfectly straight and square edges, much new lumber must be reworked.
Removal of Board Imperfections
Edge imperfections can be removed with a hand plane, a carpenter’s eye, and a lot of patience or a jointer. In the past, carpenters used hand planes and planned along the length of the board to remove the crooks. First, you need to identify where the problem areas are. This can be done by laying a stiff measuring tape along the length from end to end and marking the areas that need to be shaved down. Then you begin planing and laying a flat edge next to the board, and you will slowly see the gap disappear. This requires a lot of patience and time.
A wood jointer can quickly remove these imperfections, usually in less than a minute, if not seconds. Then when a piece of lumber is laid next to another element, they fit together smoothly and precisely. A tight ‘joint’ is formed. Hence the origin of the word ‘jointer’ a tool that can create a tight joint. The carpenters of old could only marvel at what a modern wood jointer can do in a few seconds.