An example of a polling trigger is Twitter - New tweet by a specific user:
Because of the reactive nature of polling triggers, we expect polling Applets to run within 5 minutes of a trigger event occurring for Pro and Pro+ users, and within one hour for Free users.
Realtime Applets run after the trigger service notifies IFTTT of a new trigger event. Since IFTTT is notified proactively of new trigger events, realtime Applets normally run within a few seconds of a trigger event occurring.
An example of a realtime trigger is Button widget - Button press: