By sides
Equilateral: all three sides equal. All three interior angles are 60°.
Isosceles: at least two sides equal. The angles opposite those sides are equal.
Scalene: all three sides different. All three angles are different.
By angles
Acute: every interior angle is less than 90°.
Right: one interior angle is exactly 90°. The side opposite that angle is the hypotenuse.
Obtuse: one interior angle is greater than 90°.
Working rule
The three interior angles of any triangle sum to 180°. If you know two, the third is forced.
The longest side sits opposite the largest angle. The shortest side sits opposite the smallest angle.