Some quotes I like:
A sine wave or sinusoidal wave is the most natural representation of how many things in nature change state.
A sine wave shows how the amplitude of a variable changes with time. The variable could be audible sound for example. A single pure note is a sine wave, although it would sound a very plain and flat note indeed with none of the harmonics we normally hear in nature. A straightforward oscillating or alternating current or voltage within a wire can also be represented by a sine wave.
Not only does the wave cycle from its high point +1 at -0°, through 0 at 90°, to its minimum -1 at 180°, back through zero at the closing square... it also hits +0.5 at 60° and -0.5 at 120°. The curve defines the major aspects.The sine of any angle can vary from −1 to +1. For example the sine of 0° is 0 and the sine of 90° is 1. The sine of 270° is −1 and when we get to 360° we are back to zero again.