Whenever a force is applied to an object, causing the object to move, work is done by the force. Mathematically, it is given as

W = F d cos θ where θ is the angle between the force and the displacement.  The unit of work is the unit of energy, the joule (J). 1 J = 1 N m.

Case 1:  Work can be positive

If the force has a component in the same direction as the displacement of the object, the force is doing positive work.

Consider that you are picking up a book from floor and keeping it on a table. Then work done is positive as you are taking the body opposing the gravitational force of the earth.

Case 2:  Work can be negative

If the force has a component in the direction opposite to the displacement, the force does negative work.

If you pick up a book from the table and put it on the floor, you are moving the object in the same direction as that of the gravitational force. Hence, work done is negative.

Case 3 Work done can be zero

If you just push the book on the floor with a constant velocity, so that it moves forward, then no work is done. Here, though they may be frictional force opposing the motion of the book, it cancels with the force supplied to push the book. Hence, net force on the system is zero.

The reasons why work done is zero are

1. The object does not gain any energy
2. Also, the frictional force and the force applied to push the object cancel out each other. Hence, there is no change in energy