Hello and thank you in advance for your help. I'm a beginner, creating my first website, not yet online. I cannot understand one thing. I created a .CSS file with my divs, fonts, backgrounds... but I would like a title and/or images be in the div "header" on every pages for instance, where and how should I write this text ? Will I have to copy the text in every pages of my site ?
Thank you for all your advices and help....

You can copy it on every page, or use
SSI/php includes: http://www.lissaexplains.com/html6.shtml
Javascript include: http://javascript.about.com/library/weekly/aa102901a.htm

The benefits of doing an "include" on the server are huge, almost to the point where including header information with JavaScript would be terrible idea (especially from the accessibility point of view).

Rather than use "include()" in PHP, it would be more effecient to use "readfile()". Both serve the same purpose, but readfile is best suited for including .html files (because it doesn't parse any PHP that might be in the file).