Push notifications are notifications that are passively displayed on your computer or mobile device. They can come from an app or from a website. Successful companies like CNN and Facebook use push notifications to get their visitors to come back to consume new content. App developers use them to keep people using their app and to notify people of important developments like sales and new features.
Users must opt-in to receive push notifications, so they can't be used for spam, but that's a good thing. People who opt-in have done so because they want to hear from you. In this post, we are specifically going to focus on web push notifications rather than app notifications, because pretty much anyone who has an app understands what they are and how to use them but website owners often don't even realize they can use them, much less how to do it.