Google Sheet
Google Document is the cloud-based spreadsheet format used as the native format of spreadsheets created within the office suite within Google Drive. (Google Document, .gdoc, is used for other sorts of documents.) The document content is "in the cloud"; when you sync a Google Drive directory with a local operating system, only a pointer file gets transferred to the local system, so if you don't entirely trust your documents to the cloud, or want to work with them offline without an Internet connection, you need to export them into a different format.
The .gsheet file contains a line such as this:
{"url": "https://docs.google.com/spreadsheet/ccc?key=0Ah25sBmz3CtZdEJJN3ZuQlJCM3lPVGRJQ1RocEdPNmc&usp=docslist_api", "resource_id": "spreadsheet:0Ah25sBmz3CtZdEJJN3ZuQlJCM3lPVGRJQ1RocEdPNmc"}
It points at the document content on Google's servers.
