What are those funny waves?

ddx site catalogue - sin cos tan curvesFor many years I found those curves that appeared on the old oscilloscopes a slippery thing. How were they ‘sine waves’ and what on earth did it mean when people said that two wave forms were ’90 degrees out of phase’.

It helped when it first dawned on me what simple concepts sines and cosines actually were. Bot still..’90 degrees out of phase’?

So I like this design for its liveliness, for the neatness of the concept embodied in the small icons and, not least, because when I look at it sets my mind at rest.