Workspaces
Workspaces are there to help organize your engagements and aggregate results in a meaningful way. When you create a workspace and add components to it (emails, pages, etc.), those components will only be shown and usable for campaigns in that workspace.
Deleting a workspace will also delete all objects stored within it (results, emails, pages, etc).
The General Workspace
Components placed in the general workspace will be available for use in other workspaces, but won't appear in other workspaces. This is a good place to store templates you use across engagements without modification each time.
Typically, I store all my non-client specific templates in the General workspace and then if any require client-specific edits, I'll use the import feature to copy it to my client workspace.