The difference is between Track and Desired Track.
Track is where you actually go, Desired Track is where you intended to go.
This is easily forgotton by people who use GoTo's on their GPS. Put in a GoTo with a crosswind and your track will not resemble a straight line in the slightest, nor will your time en-route be as calculated.
Any sailors who have been "sucked down the Swinge" will know exactly what I mean (and no it's not illegal or immoral
)
Mike