Tacking is when, in a sailboat, the wind moves across the bow. (that's what us sailors call the front.) Compare to jibe, which is what happens when the wind moves across the stern.

FoxtrotJuliet seems to be confused about this distiction; tacking is harmless compared to jibing. Let me explain. When you are tacking, you are probably going to windward, e.g. trying to get to the place the wind is coming from. That means the apparent wind is probably 20-40 degrees off your bow. Now, if the wind gets closer to your bow, the boat will luff - that is, the sails flap wildly. While they do that, the boom will shake - but the worst it will do is give you a bruise. The tack is only completed when you pass through the wind and head down with the sails on the other side.

Now consider jibing. In this case you're going nearly down wind. If you head all the way down and start heading up, you jibe. Note, however, that you do not luff in the middle. When the sails are ready to go to the other side, they do so on their own ... FAST. Now remember that large metal beam called the boom attached to them. This how many careless sailors meet their untimely fate.

Another thing worthy of mention is the roll tack. Due mainly to that whole luffing thing, tacking is slow. Roll tacking is faster, and is therefore the prefered method of taking 420's, FJ's, Laser's, and pretty much any boat small enough. Though it's really about feel for the boat and practice, the basics are like this: when you're about to turn through the wind, shift your weight to leward, e.g. the side of the boat that is not facing the wind. Then, while you're turning like hell, shift your weight back ... but wait, you're weight is on the leward side again because you tacked. Here's the key. Shift your weight again. This time doing so swings the sails into the wind, now all lined up on the new point of sail. This is sort of like an artificial gust, and makes you accelerate. Trust me, it's superfast.