$f = \File::getByID(1); // Retrieves the File object with an ID of 1.
This retrieves "1" – by running the getFileID() method on the $f object itself.
This retrieves "http://www.mysite.com/1234/1234/1234/foo.txt" – the URL of the file. The method getURL() doesn't exist on the \Concrete\Core\File\File object – it exists on the \Concrete\Core\File\Version object. The File object runs $this->getApprovedVersion() behind the scenes to retrieve the approved version of the File, and passes the method onto it.
$specific = $f->getVersion(5); // Retrieves the version of the file with the specific version ID of 5.
In all these cases, the returned object is an instance of a specific Concrete\Core\File\Version object. The API documentation details all of the methods available on this object.