Every Editor in Blender is divided into Regions. Regions can have smaller structuring elements like tabs and panels with buttons, controls and widgets placed within them.
At least one region is always visible. It is called the Main region and is the most prominent part of the editor.
Each editor has a specific purpose, so the main region and the availability of additional regions are different between editors. See specific documentation about each editor in the Editors chapter.
A header is a small horizontal strip, which sits either at the top or bottom of an area. All editors have a header acting as a container for menus and commonly used tools. Menus and buttons will change with the editor type and the selected object and mode.
The Toolbar (on the left side of the editor area) contains a set of interactive tools. T toggles the visibility of the Toolbar.
This is further documented here: Toolbar.
The Tool Settings (at the top/bottom of the editor area) contains as its name suggests the settings of the active tool. It’s visibility can be toggled with the header’s context menu just as its position with the Flip to Bottom/Top operator.
Adjust Last Operation
The Adjust Last Operation is a region that shows tool options when tools (operators) are run.
This is further documented here: Adjust Last Operation.
A region can be scrolled vertically and/or horizontally by dragging it with the MMB. If the region has no zoom level, it can be scrolled by using the Wheel, while the mouse hovers over it.
Changing the Size and Hiding
Resizing regions works by dragging their border, the same way as Areas.
To hide a region resize it down to nothing. A hidden region leaves a little arrow sign. LMB on this icon to make the region reappear.