Scalene triangles are triangles where each side is a different length. They are unusual in that the are defined by what they are not. Most triangles drawn at random would be scalene. The interior angles of a scalene triangle are always all different. The converse of this is also true - If all three angles are different, then the triangle is scalene, and all the sides are different lengths.
To see why this is so, imagine two angles are the same. The triangle would then be an Isosceles triangle, which has two sides the same length. Similarly, if all three angles are the same, it would be an equilateral triangle and all three sides would be the same length. See the entries for Isosceles and Equilateral triangles.
For more on this see Side / angle relationship in a triangle
For more on this see Side / angle relationship in a triangle