For a layman the term 'work' implies any activity resulting in muscular or mental exertion. In physics, however, the term has a different meaning. It represents a physical quantity.
When a force acts on an object, if the object moves in the direction of force, we say that the force has done work on the object. When you push a book on a table you exert force on the book and as the book moves we say that the force has done work.
Let us now consider a man pushing a wall. This will definitely tire him. But scientifically, no work is done, as the wall does not move.
