For many years in photography, lens flare was actively avoided by photographers. Flare, which is basically light scattered within the lens due to unplanned reflection and lens impurities, was considered as something that ruined a photograph. Equipment like lens hoods were invented to help prevent it. But times change and so do fashions.
These days, not only to many photographers not mind some lens flare, they actively seek to get it to add some creativity to their shots. And, to be fair, quite often it looks pretty cool. Check out these great examples of creative lens flare and look at the resources at the end to find out how to shoot with or without lens flare in your own shots.