When a sailing boat sails upwind (close hauled) or on a reach, the crew present the leading edge of the sails (the luff) to the wind direction at such an angle that aerodynamic effect lifts the vessel forwards and sideways.
The forward motion, we want. The sideway motion is a bi-product of the wind angle and this slippage through the water in a sideways direction is defined as leeway.
The closer to the wind a sailing vessel sails, the more leeway she will experience. On the other hand, a vessel sailing downwind will experience little on no leeway. This is because the aerodynamic bi-product is no longer there. The vessel is being blown in the direction of the wind. There are no competing vectors acting on the vessel.
Leeway varies from boat to boat, depending upon design. It also varies depending on wind strength, sail plan and sea state. An overly canvased yacht sailing upwind in high winds and a rough sea would experience very significant leeway, perhaps even making no progress as a result.
Leeway must be applied when estimating positions or calculating a course to steer. It’s therefore important to know the characteristics of your boat so that this can be properly estimated. Once you know your yachts characteristics, estimations become more accurate. In the meantime, when sailing check the wake in the water behind you. The angle upwind from parallel with your water track is your angle of leeway.
In ‘average’ conditions, sailing upwind on a 36 ft cruising yacht you might expect 5 - 10 degrees of leeway if properly canvased in not too rough a sea and with a competent helm.