In the app:
The notifications icon is shaped like a bell and is in the bottom right corner of your screen. Tap the icon to see all notifications: challenges, likes, comments, follows etc.


On the QuizUp website:

The notifications icon is shaped like a bell and is in the top right corner of your screen, next to your username. Tap the icon to see all notifications: challenges, likes, comments, follows etc.


Android users:

On Android devices, notifications are called Activity.