What it measures
Curve is the sideways distance, measured at right angles, between the ball's starting line (launch direction) and the spot where it comes back down to launch level. It is measured in feet, yards or meters.
For a right-handed golfer, a positive value or an R means the ball bent to the right (a fade or slice), and a negative value or an L means it bent left (a draw or hook). Because curve is measured from the starting line, not the target, a ball that starts left and curves right back to the flag still shows a right curve.
Typical numbers
There is no tour average for curve, because most tour players choose a shape. A controlled tour fade or draw with driver often bends somewhere in the range of 5 to 15 yards, and iron shots usually bend less.
Everyday golfers commonly see 15 to 40 yards of curve with driver on a miss, and far less with wedges. The same swing usually curves more with driver than with a wedge, because driver has less loft and the ball stays in the air longer at higher speed.
What changes it
The main cause is face to path, the difference between where the clubface points and the direction the club is moving at impact. A face open to the path tilts the spin axis right and the ball curves right. A face closed to the path tilts it left and the ball curves left. The bigger the difference, the more curve.
Strike location matters too, especially with woods. Through gear effect, a toe strike tends to add draw spin and a heel strike tends to add fade spin for a right-handed golfer. Less loft and more ball speed both increase curve, because lofted clubs create more backspin, which tilts the spin axis less for the same face to path. Crosswind can add to or cancel some of the bend.
What it changes
Curve combines with launch direction to set Side, where the ball lands relative to the target. It also costs some carry: a ball with a heavily tilted spin axis loses lift, so big hooks dive and big slices often fly weaker. After landing, the ball tends to keep running in the direction it was curving, which shows up in Side Total.
How to use it
Read curve together with launch direction. If the ball starts on line but bends offline, work on face to path. If it starts offline with little curve, the face angle at impact is the issue. A consistent, modest curve is easier to play than a straight shot that sometimes goes either way.
Bay drill: pick a target on the screen and hit ten shots trying to start each one just right of it and curve it back by 5 yards or less. Record curve on each shot. Then flip it to start left and curve right. Matching the number you call before the swing is a good test of face control.