In more practical terms, the head wind (or the headwind component) is the difference between your true airspeed and your ground speed.
In
GA aircraft at relatively low altitudes your indicated airspeed is as good as your true airspeed (unless you are flying in extreme temperature and pressure conditions). As you start flying higher you need to correct your indicated airspeed (the speed you read off your airspeed indicator) for density and temperature changes in the air you are flying through. You can do this either mentally (I forget the formula - it's been a long time since flight school) or using one of those whizwheels SlamClick mentioned. In more advanced aircraft your
TAS is already calculated and displayed for you.
To obtain your ground speed, you either read it off your onboard GPS (if available) or you can read it off your
DME if you happen to be flying towards a VOR/
DME, a TACAN or even an airfield which has got an ILS/
DME. The station does not have to be exactly on your flight path; pretty much somewhere straight ahead will do, as long as it is reasonably far from your position. While it is true that this would not give you an exact ground speed, the error will not be significant enough for you to care.
The headwind component is the difference between the two. If your ground speed is greater than your
TAS then you have a tailwind.
You can also calculate the crosswind component by looking at your drift angle and your
TAS. Your crosswind component is equal to (drift angle *
TAS/60).
From the headwind and crosswind components you can roughly figure out the magnitude of the true wind. It is approximately equal to the larger value plus one third of the smaller value. Say for example you calculated your headwind component to be 10kt and your crosswind component to be 6kt, the magnitude of the real wind would be roughly equal to (10+6/3) = 12kt.
You can also find out the direction of the wind by using a quick visualization technique on your
RMI but it's a bit of a graphical explanation and it's hard to explain with words. If you really want to know I'll look up a site that explains it well or make a sketch for you.
[Edited 2008-08-16 12:03:41]