Relief in a guitar neck, also called forward-bow, is a slight curvature in the neck bringing the center of the fretboard away from the strings. This curvature is caused by the tension of the strings pulling the headstock or peghead toward the body of the guitar. Severe relief in the neck can cause high action, uncomfortable playing, and may affect the intonation of the guitar. Unlike back-bow, a small amount of relief is usually desired in guitar setups. The reason for this is that we still want there to be enough distance between the strings and the frets to allow the strings to vibrate without buzzing on the frets. When I say small, I mean small. Relief is usually not a good thing is a guitar neck. A slight amount of relief is fine and with some playing techniques it is advantageous; however, the general rule of thumb is that we want the neck/fretboard to be flat--no back-bow and no relief. The truss rod will need to be tightened to adjust the relief or flatten the neck/fretboard. Tightening the truss rod will force the neck to flatten out and bring the center of the fretboard closer to the strings. Tightening the truss rod too much can cause back-bow. Please see the truss rod adjustment page for more detailed information about adjusting relief.
Back-bow in a guitar neck is a slight curvature in the neck bringing the center of the fretboard closer to the strings. Back-bow, in almost all cases, causes the guitar to play improperly and is almost never desired in a guitar neck. Because back-bow causes the strings to become closer to the frets, back-bow can cause string buzzing and the inability to play on certain frets. Also, intonation may be affected by back-bow. The truss rod will need to be loosened to adjust the back-bow or flatten the neck/fretboard. By loosening the truss rod, you are allowing the neck to assume a more natural shape under the tension from the strings. The headstock/ peghead will once again be pulled toward the body of the guitar. Please see the truss rod adjustment page for more detailed information about adjusting back-bow.
Remember, when adjusting the relief and the back-bow of the neck, there is no right or wrong setting. The general rule is that we want a flat neck. I have repaired and played many guitars that play perfectly with slight relief in their necks. Like the string action, this setup can change with the style of guitar and guitarist. Please see the truss rod adjustment page for more information.