Commands enable you to use Excel. There are a wide range of commands in Excel. As it is a very powerful application, there are many things you can do with. Some training and experience will help you to learn more about them and what you can do. It is impossible to give full details in an answer like this.
Excel is a Microsoft product. It has many commands, such as "Save the File", import, export, copy are all commands.
It is a set of related commands that you can have together on the ribbon in Excel.
Menu
You may be referring to functions, of which Excel has many types of.
Most Excel 2007 commands are on the menu ribbon; far too many to list here. See related links for how the Excel 2007 ribbon compares to Excel 2003 commands.The ribbon at the top of Excel 2007 and higher is a list of menu options.
task pane
ribbon commands
Some of the keyboard shortcut commands are the same, but most are different. See related links for keyboard shortcuts for MS Word and MS Excel.
A submenu command
microsoft exel
Excel 2013 uses a .xlsx extension.
yes