A triangle is a two-dimensional shape with three straight sides and three angles. This means that a triangle has exactly three sides. It is classified as a polygon because it is a closed shape with straight sides. The sum of the interior angles of a triangle is always equal to 180 degrees. Triangles are among the most basic and fundamental shapes and are studied extensively in mathematics, geometry, and physics. They are used in a wide variety of applications, from building design to computer graphics to physics simulations.