Each artist has his or her own personal art style, which is developed throughout his or her lifetime. An artist's style can change and adapt as the artist grows as both an artist and as a person.
An artist's personal style usually progresses as the artist gains more confidence through experience, expands their database of knowledge, and acquires more skill with the materials.
Artists can work in many different styles. In the past, artists were usually expected to create art in whatever predominant fashion was popular at the time. Now, artists have much more freedom to work as they choose.
While it is perfectly okay to work in a variety of styles, it is generally best to focus on the ones(s) you like best, in order to fully develop your artistic potential within that style. Dabbling with different styles is a great way to experiment, but to really flourish as an artist, you need to focus on a style or two that you feel really allows you to express your inner vision.