The Unit Circle is the graph of x2 + y2 = 1. For an angle in standard position, the point of intersection of the terminal side of the angle with the Unit Circle gives the cosine and sine of the angle. The x-coordinate is the cosine, and the y-coordinate is the sine. For example, using the Unit Circle above we see:
cos 270° = 0, sin 270° = -1 (270° passes through the point (0,-1))
see also going in circles (java-enabled browser required)