QuizUp is a trivia game where players compete against each other, answering questions from several topics. Sign up with Facebook or email to get started, then select a topic you want to compete in. QuizUp has a number of popular topics available as well as a general list. Topics include things like Arts, Geography, Logos, and Game of Thrones.
After selecting your topic, you'll be matched with another player based on your respective levels. You'll answer questions in an attempt to gain points. After each quiz, your points will be combined with a finish bonus, a power up bonus, and a victory bonus if you won the match. The total is the amount of experience points you've earned, raising your level. Your level stays within each topic, so even if I'm a Level 3 in Game of Thrones, I will start back at Level 1 when I start, for instance, Name the Flag.
QuizUp is free, though it has a couple of in-app purchases. You can buy modifications that will net you double and quadrupal experience points for $1.99 and $5.99, respectively. You can download QuizUp from the App Store now.