I'm research a term called "include"

Someone mentioned a solution to my problem as being, use an include. I have a button on an html page that most people should be able to see, but some of the visit can't. In Flash or Director I just set a variable and based on that variable either display or don't display that button.

This person said that using something called an include would let those users see the button the others would not.

How can I approach this problem? Any ideas?