Custom Events can only track URL page load events. They cannot be used to track actions that take place on a given page, like a customer clicking a button on the page.
Custom Events cannot be used to track additional variable parameters associated with the conversion event, such as passing back the value of different purchases, to be able to calculate metrics like Return on Ad Spend (ROAS). Sending variable event parameters is not yet supported with Standard Events but will be in the future.
Custom Events are easier to setup:
Once you install the Base Pixel code you can create Custom Events directly in the Ads Manager by specifying URL events. No additional code needs to be added to your site.
Standard Events require installing the Base Pixel code and Event Pixel code(s).
Standard Events are limited to 9 default labels that cannot be renamed. Custom Events can be named anything you like.