Trigonometry is a branch of mathematics that is concerned with triangles and the related measurement of various angles. Because of this, it is closely related to geometry. Terminology that is typically associated with the field of trigonometry includes: sine, cosine and tangent.
The following academic lectures provide a solid overview of trigonometry. After you watch these videos you will have a good grasp on this branch of mathematics, as well as how it relates to other forms of math.