Because the names are similar, it's easy to confuse document workspaces and
document libraries, but they are very different.
A document workspace is a special type of subsite that contains
works-in-progress. Workspaces often contain only one document (perhaps a large
report or spreadsheet) that a team is working on.
A document library is used to store multiple documents within a
site. Completed documents should be stored in document libraries that are part
of the team site. Document libraries are a special type of list (not a type of
site). Libraries collect related documents and share them with all members of
the site. Documents can be organized into folders within the library, allowing
you to categorize them.
Every document workspace site actually includes a document library for the
work-in-progress, but because document workspaces are subsites of the team site,
not everyone has access to those files. Storing completed documents in a
library at the team site makes those documents more widely available.
You can't create document libraries from the Shared Workspace task pane--you
do it by viewing the SharePoint site in a browser and choosing Create
-> Document Library and completing the New
Document Library form. Of course, there's always an exception: you
can create picture libraries using the task pane in Office Picture
Manager.