As you may know, Microsoft spent a lot of money few years ago before releasing their Office 2010 suite. One of the most famous feature was the 'ribbon'. Some people like it, other ones don't, but that ribbon is there to stay.
Still sometimes someone would like to change the look and feel of the ribbon. Let's say that you need to hide a button in the ribbon from an application page.
The easiest way I've found, in my humble opinion, for achiving this result is via a css style tag within the application page.
You could use the developer tools (F12) in IE and look for the ID of the element you need to hide.
Furthermore you may find here the Default Server Ribbon Customization Locations
Those are the information you need.
Thus your css should look like this one: