A hobby is a regular activity that is done for pleasure, typically, during one's leisure time. Hobbies can include: collecting themed items and objects, engaging in creative and artistic pursuits, and playing sports, along with many more examples. By continually participating in a particular hobby, one can acquire substantial skill and knowledge in that area.
Generally speaking, a person who engages in an activity solely for fun is called an amateur (or hobbyist), as opposed to a professional who engages in an activity for reward. An amateur may be as skilled as a professional, the principle difference being that a professional receives compensation while an amateur does not.