The active cell is recognized by its black outline. Data is always entered into the active cell. Different cells can be made active by clicking on them with the mouse or by using the arrow keys on the keyboard.
The File tab is new to Excel 2010 - Sort of. It is a replacement for the Office Button in Excel 2007 which was a replacement for the file menu in earlier versions of Excel.
The Ribbon is the strip of buttons and icons located above the work area. The Ribbon is organized into a series of tabs - such as File, Home, and Formulas. Each tab contains a number of related features and options. First introduced in Excel 2007, the Ribbon replaced the menus and toolbars found in Excel 2003 and earlier versions.