What's the difference between the three?

Views: A view is an element that is displayed or contains other views that can be displayed, only on iOS. An entire screen can be a view, and so can a big list on the screen. For an ecommerce app, item blocks themselves are views, and the item picture is also a view.

Activities: Activities are individual sections of an Android app, similar to views, but they can encompass larger portions of the app with multiple screens. For example, PaymentActivity may refer to the entire payment process with multiple screens, and MainActivity may wrap the entire app.

Fragments: Fragments are smaller sections of Android activities and usually represent a certain specific screen the user is on. For example, a final payment submission form may be a specific fragment within the Android PaymentActivity. We do not automatically track fragments like we do views or activities, but users can create breadcrumbs to track fragments. Note these breadcrumbs will appear in the user timeline, but can't be used to track details like Last View.