What are quick actions in Salesforce, and how do they differ from custom buttons?

Quick Actions are customisable actions that appear in the action bar of a Lightning record page or on the Chatter feed. They open a compact action layout (not a full page layout) in a modal dialog.
Types:
— Object-specific actions: Tied to a specific object and appear when viewing records of that type.
— Global actions: Available across the entire org (from the global header's New button).
Action layouts: A separate, trimmed-down layout defined specifically for the quick action modal — typically 4–8 fields rather than a full page layout.
Predefined field values: You can set default values for fields in a quick action (e.g., pre-populate the Status field when creating a Task from a Contact).
Custom buttons (legacy): Traditional buttons defined in Setup with URL formulas or JavaScript. JavaScript custom buttons are not supported in Lightning Experience (deprecated). All JavaScript button logic should be migrated to LWC actions or Screen Flows. URL-based buttons still work but have limited capability compared to flows.