A sport is defined as an activity that is competitive, physical and organized. Sports are leisure activities and are not generally considered work, with the exception of professional sports.
The primary defining characteristic of sports is that there must be competition, either against and opponent or against one self. For an activity to be competitive, there must be the possibility for one of the participants to be declared the victor. Without competition the activity is simply leisure.
In addition to being competitive, sports require physical exertion. The physical exertion can vary from extreme, in the case of a marathon runner, to light, in the case of a golfer. The important thing is that some level of physical skill is being used.
For an activity to be considered a sport, it must also be organized. A single runner covering the distance of a marathon is not a sport. The runner is engaging in play, or a leisure activity. When other runners are present, on a specific day and time, and the runners compete to see who can cover the distance the most quickly, the event is considered a sport.
Finally, sports are not work. With the exception of professional sports, sports are activities that people undertake for pleasure.