A triangle is a three-sided polygon characterized by its three vertices and three edges. The sum of the interior angles of any triangle is always 180 degrees. Triangles can be classified based on their angles: acute triangles have all angles less than 90 degrees, right triangles have one angle equal to 90 degrees, and obtuse triangles have one angle greater than 90 degrees. Triangles can also be categorized by their sides. An equilateral triangle has all three sides equal, while an isosceles triangle has two sides of equal length. A scalene triangle has all sides of different lengths. The properties of triangles are fundamental in geometry and are used in various applications, including architecture and engineering.