Is angular momentum a scalar or vector? If it is a vector, what rule is used to determine its direction?

Hint: Terms, vector and scalar, having only a minute difference in understanding these terms brings about great change in various aspects of physics. Relying on two factors, magnitude and direction, basic knowledge is required to understand the question. Basic knowledge of angular momentum is required to answer the question.

Complete answer:
Angular momentum is a substitute for linear momentum but in rotatory motion. Linear momentum describes the motion of an object under force. Similarly Angular momentum describes the inertia of rotation about the imaginary or actual axis either passing through or away from the object itself. It is of two types based on the axis of rotation:
1. Spin angular momentum (momentum due to rotation)
2. Orbital angular momentum (momentum due to revolution)
Angular momentum is an analogue of linear momentum, so it is also a vector quantity (i.e. dependent upon both direction and magnitude). Due to this property of angular momentum it becomes necessary to not only know its magnitude but also its direction. Direction of angular momentum of the spinning body or rotating body is determined by the help of the Right-hand thumb rule.
Right hand thumb rule is described to evaluate the direction of angular momentum. According to the rule, if we stretch open our palm, pointing all the fingers towards the axis of rotation or representing the radius of rotation and then folding the fingers in the direction of rotation of the object, at that time the direction in which the thumb points is the direction of angular momentum of the that object.

Note:
We should never forget that in rotatory motion terms like angular momentum and angular velocity are always in a direction perpendicular to the plane of rotation. Angular momentum is the sum of both spin and orbital angular momentum.