There are 7 types of triangles classified by their sides and angles: equilateral triangles have 3 congruent sides; isosceles triangles have 2 congruent sides; scalene triangles have no congruent sides; equiangular triangles have 3 congruent angles; acute triangles have 3 acute angles; obtuse triangles have 1 obtuse angle; and right triangles have 1 right angle. The lesson teaches how to classify triangles based on these properties of their sides and angles.