The angular momentum of a body is defined with reference to a chosen point also called the origin. Angular momentum of a body is a cross product given by L = r x p, where r is the position vector of the body about the point chosen as origin and p is the linear momentum of the body. As linear momentum is a cross product of two vectors, it is a vector too and acts in a direction that is given by the right hand rule.
Angular momentum is expressed in the units N*m*s.
The angular momentum of a rotating body has many applications; a gyroscope does not fall over due to its angular momentum, a moving bicycle is stable and does not tip over due to the angular momentum of its wheels.