What is the definition of work when net force is parallel to the distance?

Work in general is given by W=F·d where F is the force vector and d is the displacement vector.  The dot symbol is the dot product which is a measure of how parallel two vectors are.  It can be replaced by the cosine of the angle between the two vectors and the vectors replaced by their magnitudes.  If F and d are parallel then the angle is zero and the cosine is unity.  So in this case work can be defined as the product of the magnitudes of the force and distance:
W=Fd

More information