Fixed UI
A fixed UI is any kind of UI that is always visible to the user. The canonical example for this is the application menu, and the toolbar.
Google Docs Toolbar:
Pros
Okay discoverability.
The toolbar and menus are always visible. The user knows where to search a particular action. Actions can be organised under groups and labels. But it can be hard to find something in so many options.Excellent coverage. Most actions can be implemented through buttons and menus. Complex and rarely used actions can be made available within a menu, without cluttering the UI (ex: "Search")
Cons
Mediocre context. Actions are far from the thing they act on. If the meaning of a button changes according to the context, it can be really confusing. Actions can be disabled or hidden depending on the context, but that can be hard to notice.
Slow to use. Needs to use the mouse, to navigate through menu or dropdowns. Complex actions may require extra steps ("Add image..." that spawns a modal).
Visual clutter, can have too much information. It distracts a lot and is not beginner friendly.