A method written in C# for calculating the cross product between two vectors:
Vector3d CrossProduct(Vector3d v1, Vector3d v2)
double x, y, z;
x = v1.Y * v2.Z - v2.Y * v1.Z;
y = (v1.X * v2.Z - v2.X * v1.Z) * -1;
z = v1.X * v2.Y - v2.X * v1.Y;
var rtnvector = new Vector3d(x, y, z);
The cross product enables you to find the vector that is ‘perpendicular’ to two other vectors in 3D space. The magnitude of the resultant vector is a function of the ‘perpendicularness’ of the input vectors.
Read more about the cross product here.