I'm trying to get a grip on some classic physics by watching a Stanford lecture. I've made it through the first one, and now in the second one all the professor ever talks about is 'derivatives' without actually explaining what they are. I guess the Stanford population would be assured with some previous math knowledge. So anyway, apparently we can work out velocity from position with v = dx / dt, that's velocity equals position over time. Makes sense. We can further work out acceleration with dv / dt, velocity over time. Now why on earth are there these d's in front of the variables? What exactly is meant by the word 'derivative', and what is a vector? If these questions are too naggy, and I should perhaps be looking at some basic maths before I look at classic physics, could anyone recommend some good starting points (wikipedia is useless for this sort of thing).