A truss rod is a rigid shank that runs down the length of a guitar neck under the fret board. It can be made of wood or graphite, but is more commonly made of steel. This inner rod helps counterbalance string tension on the neck, keeping the neck of the guitar relatively flat. Some guitars, such as Rickenbackers, have dual truss rods for extra stability.
Truss rods are adjustable so that neck bow (known as "relief") that might occur over time can be countered. Seasonal changes, humidity and switching between heavier and lighter gauge strings can all affect the guitar neck. The truss can only be adjusted from one location that varies on different models of guitars. In most cases the adjustment head is hidden behind a small plate, but on other guitars it is readily visible.
Electric guitars with bolt-on necks generally feature the rod adjuster at the base of the neck hidden under the pickguard plate. Models with this setup include older Fenders® and Fender-style copies, many Yamaha® guitars, and other models including vintage re-issues.
Newer Fender guitars place the adjuster at the base of the headstock just above the nut. The headstock is the portion of the guitar where the tuners or keys are located, and the nut is the small piece that guides the strings off the headstock. The truss rod adjuster is uncovered on these models, but recessed (as always) in a concave hollow.
Set-neck guitars, or guitars whose necks are glued on with adhesive rather than bolted on, feature the truss rod adjuster in the same location as newer Fenders. In the set-neck models, however, the adjuster is covered with a small plate fastened by screws. Gibson® and Epiphone® guitars fall into this category.
There are two different locations possible for the adjuster on steel-stringed acoustic guitars. It will either be located at the base of the headstock as noted above, or accessible through the sound hole on the inside heel of the neck. In the former case the truss rod will be covered with a plate, but if located in the sound hole, it will be readily visible.
While electric and steel string acoustic guitars are built with truss rods, classical guitars don’t require them. These guitars use nylon strings that don’t exert the same degree of tension on the necks as steel-stringed guitars, making reinforcement unnecessary.
While truss rod adjustment is not terribly complicated, doing it incorrectly can be an expensive lesson and can ruin your guitar. If you aren’t comfortable adjusting the neck of your guitar yourself, don’t hesitate to take it to a shop. Professional adjustment is an inexpensive service.