Quaternions sound intimidating, but they are extremely easy to use. You may want to review the theoretical math behind them by viewing the following videos:
- Fantastic Quaternions by Numberphile: https://www.youtube.com/watch?v=3BR8tK-LuB0
- Understanding Quaternions by Jim Van Verth: http://gdcvault.com/play/1017653/Math-for-Game-Programmers-Understanding
However, we won't cover the math background here! In fact, you don't need to understand much about the math background of quaternions to use them effectively.