Triangles can be classified based on side lengths or internal angles. There are three types based on side lengths: equilateral (all sides equal), isosceles (two sides equal), and scalene (all sides different). There are also three types based on angles: right (one 90° angle), obtuse (one angle over 90°), and acute (all angles under 90°). Equilateral triangles meet criteria for both equilateral sides and acute angles.