Trigonometry deals with relationships between the sides and angles of triangles. It uses trigonometric functions like sine, cosine, and tangent. Sine relates the opposite side to the hypotenuse, cosine relates the adjacent side to the hypotenuse, and tangent relates the opposite side to the adjacent side. Trigonometry can be used to find unknown side lengths or angles in triangles, including solving "story problems" where trigonometric applications are needed to determine real-world quantities like shadow lengths. Key aspects of using trigonometry include identifying what quantities are given and needed to be found, setting up the appropriate trigonometric function, and drawing a right triangle to visualize the problem and solve it.