Don't use the word "layers" when referring to HTML
"Layers" was the name given to the concept of layered or stacked content on HTML pages by Netscape some time ago. They had the <layer> element which they introduced in version 4 of their browser product back in the 20th century. It was incompatible with Internet Explorer and with the standards set forth by the W3C.
Consequently support for "layers" was dropped when the Netscape product was dropped (see: netscape). They were never supported by any software other than Netscape 4 and so died along with it.
Instead of saying "layers" and invoking nightmares about <layer> elements, you can now refer to this concept simply as "an HTML element". Realize that you can position and stack any HTML element arbitrarily on a page using CSS. For further information, see Phrogz's CSS library or ask about absolutely positioned elements.