How do I Get Info for multiple items in one window?

When selecting multiple items in the Finder and hitting the ⌘+I shortcut, OS X produces one window per file, which is painful when attempting to view a summary of, for example, the total size of a number of files at once.

How can I view info for multiple files in a single window?

There are two modifier keys that allow you to view grouped info for multiple files (note that the names are derived from what the Finder's File > Get Info menu item changes into when the relevant modifier key is pressed):

• ⌥+⌘+I (alt/option + cmd + I): This opens the Inspector, an independent window that tracks info for whatever the selected files in the focused Finder window are. The Inspector updates as you change selection (or change which Finder window is focused), and remains until it is closed.
• ^+⌘+I (control + cmd + I): This opens a Summary Info window, which aggregates info on multiple selected items. Unlike the Inspector, however, it is static and always displays info on whatever items were selected when it was opened, regardless of changes to which items are selected in the Finder, or which Finder window has focus.