You know push notifications. The buzzing in your pocket when there’s breaking news. The message that pops up on your smartphone screen about a one-day sale (whether the app’s open or not). The haptic hum on your wrist alerting you to yet another flight delay. We’ve been sending and receiving these hot-now messages since 2009, and there’s no slowdown in sight. Recent innovations make push more versatile than ever, so it’s a good time to get a refresher on the basics.