Orianna is a unique champion that focuses her abilities on the use of a ball she carries with her. The ball allows you to pull off some pretty cool tricks, including slowing your opponents in an AOE around it and harassing with it during the laning phase.
If you want to know how to build Orianna's masteries, items, or runes, take a look at this champion spotlight by Riot Games for a full breakdown of her abilities and how to play her.
League of Legends is a free to play game PC game. Visit their website to try Orianna out.