Both are buttons that let users do something in one click, but they live at different levels.
A Quick Action is bound to a specific object — an "Update Status" action on Case, a "Log a Call" action on Opportunity. It can be a Create-a-Record, Update-a-Record, Log-a-Call, Send-Email, or Custom (Visualforce/LWC) action. It appears on that object's record pages and respects that object's record types and field-level security.
A Global Action isn't tied to one object. It appears in the global header (the "+" button in the utility bar) and lets a user create a record from anywhere — typically a quick "New Task", "New Event", or "Log a Call" without navigating to the parent record first. Global actions can pre-populate fields via URL parameters or via the Action's predefined values.
In Lightning Experience, Global Actions appear in the Global Publisher Layout, while Object-Specific Actions appear in either the page layout's Mobile and Lightning Actions section or via the Highlights Panel. Both types are configured in Setup -> Object Manager (object-specific) or Setup -> Global Actions (global).