It is not very clear after read several tutorials , well 2 tutorials . How BODY works in css,
I see the BODY tag being used in the css external fila as well and in HTML files . could not fin help in this topic .
Please provide a resourse to clarify this.
nazariocampos@yahoo.com cancun mexico naz_campos@hotmail.com

It sounds like you misunderstand what CSS is. It doesn't replace HTML. It adds style to HTML.
It does that by using selectors that match already existing HTML structures. That may or may not include BODY. If you have a style rule like this

body { color: black; background: yellow }

that means that all text on the page should be black on a yellow background unless it's overridden by something else, for example style rules for headings.

Read the introductory chapters in the CSS rewference.
http://htmlhelp.com/reference/css/

It does not matter where do you place your css code, external file or your html document, it has exactly the same effect. I always prefer using external css files so making changes are a lot more easier because you have to change only one file.