The loft of a golf club is the angle at which the leading edge of the clubhead strikes the ball at address. The higher the loft, the "farther back" the head will be upon impact and thus, theoretically, launch balls higher into the air.
Loft and lie are interrelated in that changing either one changes both the angle at which the clubhead strikes the ball and how that translates into distance.
Loft is one of the most important considerations for a golfer when choosing clubs, particularly since it will affect both shot distance and trajectory. Because of this and because loft is an adjustable characteristic on many clubheads (a feature known as "launch angle"), its importance on a particular shot can vary over time.
A short game with low loft is often better for "controlling" the ball's flight and direction, such that the golfer can make adjustments to alter the ball's trajectory mid-flight. This would be preferable in situations where accuracy is more important than distance (such as bunkers or short chip shots).
Higher lofted clubs are better for launching the ball longer distances with flatter trajectories. This can be advantageous when hitting long-distance shots, especially ones that require accuracy (such as fairway driving and mid-range iron/wood shots). These extra yards carry major advantages in golf because being closer to the green reduces the opportunities for the hole to become harder for longer shots.
How does loft affect distance?
The angle at which the club strikes the ball has a significant impact on both how far it goes and its trajectory. A low-lofted club will hit the ground behind the ball and launch into it, spiking low shots at relatively shallow angles of ascent.
The higher the loft, the more "vertical" the launch angle and the further propelled forward into air.
How does loft affect trajectory?
A draw shot is one that curves to the right for a right-handed golfer. In order to cause this curve, the ball must be hit on a trajectory that curves to the right (often referred to as "hooking"). Similarly, a fade is a curve in the other direction.
The loft of the club head itself doesn't affect ball trajectories directly but rather determines how high or low shots land and, therefore, on what type of trajectory they will travel from there.
High-lofted clubs promote more upward launch and thus flatter trajectories, while low-lofted clubs promote downward launch with steeper angles of ascent.
Low loft can be beneficial when aiming for a straight shot on a long distance, whereas high loft is better for shorter distances with added hooking potential to compensate for the loss in control.
However, loft is only one factor in determining a club's trajectory. The other major determinant is the lie angle of the shaft.
What loft should I have on my driver?
There is some controversy as to the proper loft for a driver. Most experts agree that 10-degrees is ideal, with lofts on either side providing distance loss or gain from there.
Some people maintain that 12-degrees of loft is easier to control and therefore better for beginners than 10, but this is not necessarily so because lower loft means more physical effort is required to launch the ball as high.
Many manufacturers will offer very low (often 8-degrees) or very high (up to 15-degrees, then "draw" versions of even higher lofts are available) driver loft options, and these may be preferred by experienced players using irons for driving.
How does loft affect accuracy?
Low loft is more accurate on shorter shots because it promotes less curvature and is easier to control. A ball that "hops" or curves at the end of its flight will be much harder to hit than one which flies straight, so low-lofted clubs are preferred for hitting layups and short chip shots.
High loft promotes a steeper angle of ascent and can result in a shot that "fades" or curves to the left (for a right-handed golfer) at the end of its flight, especially for those with slower swing speeds.
This trajectory is less desirable on shorter shots, but higher lofted clubs may be preferable for longer distances when accuracy is of greater concern than distance.
The loft of the club is a major determinant in what shots are most effective for which ranges. Lower loft is generally desirable in shorter distances because it promotes less curvature and more control. Higher loft will produce longer, harder shots whether distance or accuracy are priority but will be trickier to hit without a perfect impact time.
Loft is especially important in the irons, which are designed to strike the ball with a sharp downward angle. Low-lofted iron heads will "push" or slide under the ball, resulting in thin shots that go long but have little height and poor distance control. Higher loft promotes a steeper descent for more power and greater distances.