Googles Spreadsheet API uses the spreadsheet key. This KEY is shown in the URL when you open a spreadsheet in the browser. To see it being used, there is a simple google demo CellDemo.java. http://gdata-java-client.googlecode.com/svn-history/r51/trunk/java/sample/spreadsheet/cell/CellDemo.java
I assume it is possible to get all keys from the drive api, because you can do this:
Drive driveService = new Drive.Builder(TRANSPORT, JSON_FACTORY, credential).build();
File file = driveService.files().get(this.spreadsheetKey).execute();