The best way to visualize sailing to windward is imagine yourself crawling up a very slick, steep, icy hill. Going down is easy: Gravity (the wind) does it for you. You can go sideways pretty easily, neither up or down. If you try to go straight up, you lose traction and slide backward. But if you go up at an angle, not quite straight up, you can climb higher by zig-zagging. This is called tacking. The angle varies from boat to boat and with conditions, but for most modern yachts it's about 45 degrees on either side of dead to windward. In general, a yacht can sail in any direction except for the 90 degrees centered on the wind direction.

The old square-rigged sailing ships weren't as good, they were restricted from sailing closer than 70 degrees to the wind. But they were optimal for high speed sailing in the trade winds, which pretty much blow in one direction all season, all the way across oceans.