A Class falls within your normal booking window rules, whereas an Event does not.
Let's say you have a 2-week booking window, and it's currently July 1. Customers can book 14 days out until the middle of July. Any classes on the schedule during that 14 day period will be bookable. A class on July 16th would not be.
But let's say you have a special event you want to create at the end of August, an end-of-Summer extravaganza. It'll be on August 31st. As long as you create it on the schedule and add it as an Event, you can start to promote it now in early July. Your customers will be able to book in and buy their spot in your Event without being stopped by the normal 2-week booking window.
In order for Events to work properly, three things are required:
1) You have to have an Event Series Type already created. Click here to learn how to set up an Event Series Type and Series.
2) You must choose to create the Event as an Event on the Scheduler.
3) You must choose the proper Event Series Type(s) when setting up the Event on the Scheduler, so your customers can book in properly.