An app activity custom audience is an audience used for ad targeting of people who launched your app or took specific actions while using it.

Within your ad set, you can further narrow targeting by adding a connection to a page, app, or event that you control.

A customer list custom audience is created by uploading a customer list that Facebook matches with users so you can target them with ads.

You can create an Event custom audience to target those who have engaged with your Facebook event with an ad.

Facebook page custom audiences allow you to create an audience of people who interacted with your page to be used in ad targeting.