In Views, what is the difference between "Show Fields" and "Show Content" under Format?
Show: content will render the content as it is normally displayed if you access it at its URL.  For example, selecting Show: content for a view displaying users will display each user's full profile.  Selecting it for a view displaying nodes will render each node in its entirety.

Show: fields, by contrast, allows you to select (limit) which fields to display.