Based on ship's data (heading and speed over ground), this nautical wind calculator returns true and apparent wind (speed and direction). Anemometers on board of a ship often measure apparent wind. This app lets you quickly convert to true wind.
You can also convert true wind to apparent wind. The calculator has a help page which clearly explains all symbols used.
The power of this calculator is the graphical solution or plot, clearly showing how wind vector and ship movement vector are added as a vector sum. Make sure you understand this plotting technique very well.
In the theory section, the Beaufort scale of wind force and effects of wind on the sea are explained. This will help you estimating true wind direction and wind speed based on the state of the sea.
This section ends with some useful formulas to convert between true and apparent wind.