Story: As a user I want to know what happens after I click a button.
Description: Buttons can trigger arbitrary things, however, there are several common patterns:
- Execute an action directly (like triggering a save that goes directly into a database) standard case
- Show a popup (like the one that your OS usually shows before printing) usually signified by "…"
- Expand an element in the UI, like a dropdown, an additional toolbar etc. can be signified by a chevron
Related: T44138 (VisualEditor: Toolbar "Save page" button is confusing as it merely opens the dialog to save the page)
Microsoft has a nice list of button actions and how they can be signified