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 measurements. The document provides examples of using trigonometric functions like sine, cosine, and tangent to solve for unknown values in right and non-right triangles, as well as how to set up and solve word problems involving trigonometric applications.