There are several ways to classify triangles, but two common methods are by their side lengths and by their angles.
Classification by side length:
Scalene triangle: a triangle with no congruent sides
Isosceles triangle: a triangle with two congruent sides
Equilateral triangle: a triangle with three congruent sides
Classification by angle measure:
Acute triangle: a triangle with all angles measuring less than 90 degrees
Right triangle: a triangle with one angle measuring exactly 90 degrees
Obtuse triangle: a triangle with one angle measuring more than 90 degrees
It is worth noting that a triangle can be both classified by its side lengths and angles, for example, a right isosceles triangle is a triangle with one right angle and two congruent sides or it can be a right triangle with all congruent angles.