The cotangent of an angle is the reciprocal of the tangent of an angle. The cotangent of an angle on the unit circle corresponds to the section of the line tangent to the point formed by the angle and intersecting the y-axis.
The graph of cotangent and tangent shows the reciprocal relationship between the two functions. As tangent approaches so does cotangent and as tangent approaches cotangent diverges to infinity.
See the rest of the trigonometric identities on this page.