As children, we are free of responsibility and may do almost anything we like in whatever fashion suits us. This is called playing. As we grow up, we must become much more focused on our obligations at home, school, and work. However, as "All work and no play makes Jack a dull boy," we still need recreational outlets.
Hobbies provide the perfect such outlets. Hobbies give people an escape from their responsibilities and allow them to enjoy themselves. Hobbies are a mature mixture of work and play, as they allow participants to develop skills while doing something they enjoy.
Throughout the course of a person's life, one may have many hobbies (as one's interests change). Perhaps a boy builds model trains when he is ten, but restores old airplanes as a forty-five year old man. Maybe a girl of six collects dolls, then learns aerobics at twenty-five. Hobbies are entirely individual.
My personal hobby is practicing martial arts. I find martial arts valuable for training and strengthening the body, while also allowing a tremendous outlet for energy and frustration. Martial arts also develop practical skills for self-protection. Most importantly, they allow one to set long-term goals, which can be very meaningful to one's life.
Whatever your interests, hobbies can be very enjoyable. Choose a hobby that appeals to you, and strive hard in this "realm between work and play."