The applet below draws the graphs of the primary trigonometric functions and shows their relationship to the angle formed in a unit circle. Select one or more of the functions and select Trace it to draw the graph as the angle progresses through 720º . Notice, in the unit circle, that the sine just corresponds to the y- coordinate of the point on the circle. Similarly, cosine is the x- coordinate of the point on the unit circle.
Tangent, on the other hand, comes from the length of the segment on the tangent to the circle that is on the line through the centre of the circle and the point on the circle.