For questions about the web version of Google Documents, the word processor from Google. For questions about the file manager application use tag [google-drive] instead. Do NOT normally apply both to the same question.
Google Documents is part of Google Drive and allows the user to create new documents on the web interface (as opposed to using a local word processor software). Documents on Google Drive can be shared with others to collaborate, or simply used as a document editor.
(* to indicate that you want to convert the document to PDF in Google Drive)