WORK may be defined as activity involving mental or physical effort done in order to achieve a purpose or result.
PLAY may be defined as activity involving mental or physical effort done for enjoyment and recreation rather than a serious or practical purpose.
So, as Mark Twain said, "Work and play are words used to describe the same thing under differing conditions."
Notice that the essential way that work differs from play is by the purpose and by the degree of seriousness.
Following are notes from 'WORK' by Bertrand Russell.
Whether work should be placed among the causes of happiness or among the causes of unhappiness may perhaps be regarded as a doubtful question.
There is certainly much work which is exceedingly irksome, and an excess of work is always very painful.
I think, however, that, provided work is not excessive in amount, even the dullest work is to most people less painful than idleness.
Most of the work that most people have to do is not in itself interesting, but even such work has certain great advantages.
To begin with, it fills a good many hours of the day without the need of deciding what one shall do. Most people, when they are left free to fill their own time according to their own choice, are at a loss to think of anything sufficiently pleasant to be worth doing. And watever they decide on, they are troubled by the feeling that something else would have been pleasanter. To be able to fill leisure intelligently is the last product of civilization, and at present very few people have reached this level. Moreover the exercise of choice is in itself tiresome. Except to people with unusual initiative it is positively agreeable to be told what to do at each hour of the day, provided the orders are not too unpleasant. Most of the idle rich suffer unspeakable boredom as the price of their freedom from drudgery. The more intelligent rich men work nearly as hard as if they were poor, while rich women for the most part keep themselves busy with innumerable trifles of whose earth-shaking importance they are firmly persuaded.
Work therefore is desirable, first and foremost, as a preventative of boredom, for boredom that a man feels when he is doing necessary though uninteresting work is as nothing in comparison with the boredom that he feels when he has nothing to do with his days.
With this advantage of work another is associated, namely that it makes holidays much more delicious when they come. Provided a man does not have to work so hard as to weaken or damage his health, he is likely to find far more zest in his free time than an idle man could possibly find.