First understand that no amount of club adjustment can cure your swing flaws. However, recent innovations in club technology allow drivers, fairway metals, and hybrids to be adjusted to suit or counteract a player's swing tendencies. If a player tends to hit very high tee shots, he or she may want to lower the loft on their driver.
Too low of a tee ball will require loft to be added. If, however, the player tends to hook the ball, the face angle can be opened, a tweak which makes it more difficult to impart hook spin to the ball. The opposite is true for a slicer: closing the face angle will counteract slice spin.
If you are considering buying an adjustable club, it is probably wise to begin with a driver. Since the driver has the lowest amount of loft of any club besides your putter, sidespin is maximized on miss-hits, therefore some adjustment to the clubhead will often help to straighten out those all-important tee shots.