The sine and cosine functions are fundamental concepts in trigonometry, which is the study of triangles. They relate the angles of a right triangle to the ratios of its sides. For a given angle, the sine is the ratio of the length of the opposite side to the hypotenuse, while the cosine is the ratio of the length of the adjacent side to the hypotenuse. These functions are also represented on the unit circle, where the angle corresponds to a point on the circle. The sine value is the y-coordinate, and the cosine value is the x-coordinate of that point. Both functions are periodic, meaning they repeat their values in regular intervals.