Consider the simplest possible case of work done. A force F is acting on an object. The object has a displacement S in the direction of the force. Then the work done is the product of force and displacement.
What will happen in the case when the applied force is not in the direction of displacement but rather at an angle to it. In such a case we will consider the component of force in the direction of displacement. This component will be effective in doing work as shown.
Component of force in the direction of displacement is F Cos θ.
W = (F Cos θ) S = F. S
Work done is a scalar quantity.
SI unit of work is Joule or J
1J = 1 Nm
Where N is the unit of force and m is the unit of displacement.
Also 1J = 107 ergs. Where ergs is the cgs unit of work.