The tension in the strings exerts a huge bending force on the guitar neck, and combined with environmental conditions like temperature and humidity can cause bowing of the neck. Most guitars have an adjustable truss rod, running the length of the neck that can be adjusted to counteract the force caused by the pull of the strings, as well as strengthening the neck and help stabilise the necks straightness. If your truss rod is too loose, it will result in a concave neck bow, (action too high) and a truss rod that is too tight will result in a convex neck hump (action too low and causing fret buzz).
The truth is that the truss rod is a simple device that has one purpose: to counter the pull of the strings. That's it. It is NOT meant to adjust the height of your strings; you can NOT set your intonation with it.

There are a few golden rules to obey when adjusting a truss rod. First, only use the rod to keep your neck as straight as it needs to be…do not use it to adjust your action! Second, only use the proper adjustment tool. If you do not know what this is, check with the manufacturer or visit a good tech. Finally, do not force anything; an eighth of a turn can make a good amount of change.
The first step is to know in what direction the neck needs to be adjusted.
Start by sitting with the guitar in the playing position. Make sure it is in tune and capo at the first fret. If you do not own a capo, you can simply fret the strings, but this may make things a little more difficult. Next, fret the bass string at the 14th fret. You are using the string as a straight edge to read the curve. Check the height of the string over the 6th fret. Gently taping the string to the fret makes the size of this gap clearer.
The bigger this gap the more bow in the neck. It's a good idea to check the centre strings and treble strings as well to give you an idea of the neck overall. It is possible to have a neck that is straighter on one side than the other. If there is no gap at the 6th fret, your neck is either dead straight or has a Convex - hump (back bow). A buzzing of the strings open and within the first two frets is a definite indication of back bow. If you are touching at the 6th, let the string go and see if there is a gap at the first fret. The greater this gap, the greater the amount of back bow. Having a straight neck makes for a better playing instrument, but not all guitars and playing styles allow for a completely straight neck.
Therefore, some relief is acceptable, and often necessary.
Due to the elliptical vibration of a string, a little relief may prevent buzzing in the first and second positions. If you have a strong picking hand and tend to be an aggressive player, you can exaggerate the movement of the string and may need some relief, and higher action. Players with a lighter touch often have straighter necks with lower action.
Electric guitars usually have straighter necks than acoustics, and basses typically have more relief than guitars.
Be aware that straightening the neck may seem to make your guitar play worse. Tightening the truss rod straightens the neck and consequently lowers the strings, which can create string buzz. However, string height is controlled at the nut and saddle, not in the neck. Likewise, a straight neck may also make a poor fret condition more noticeable. Ultimately though, a good straight neck is the first step in a proper set-up and should help to make your guitar play better. If a straight neck makes the guitar play worse, the neck is either too straight for your playing style or it is a sign that more work is needed.

Once you have a picture of the neck as it is, you need to decide what way to turn the truss rod nut and how much to turn it. Remember in a single action truss rod: tightening the rod (turning clockwise) straightens the neck, loosening (turning anti-clockwise) permits it to bow. Before you adjust the nut, make a mark on it that corresponds to a fixed point below it on the access channel to the nut. This will help you gauge how much you have turned and may help you get back to "0" if for some reason you get too far off.
Once you are prepared to make the adjustment, keep in mind the gap you read at the 6th fret. The greater the gap, the more you will tighten the rod. If you had no gap you will be loosening the rod. Keep in mind that most good necks require no more than half of a turn in either direction. Using the mark you made to note the original position of the rod, start with an eighth of a turn, re-tune the guitar and check the gap again. Keep tightening or loosening, then re-tuning until you can just barely see light through the gap at the 6th fret, but remember not to force anything. Stop if you are applying a lot of torque, and the neck still isn't going straight. The adjustment should be fairly smooth and easy.
Once you have the neck straight, play the instrument and see what effect the adjustment had. If buzzing is apparent in the first few frets, try loosening the truss rod slightly, re-tune and see if it helps. If you have buzzing consistently up the neck, you are most likely in need of a good set-up and/or fretwork.
The procedure outlined here is very basic. It is intended for the typical player, and could easily be much more in-depth than this. A good tech would be a lot more precise, while still following a similar procedure. However, from a maintenance standpoint, this is as technical as it needs to be. This same process holds for any stringed instrument with a single action adjustable truss rod. As long as you follow the basic rules, adjusting a truss rod should be as simple and routine as changing your strings.