Four tier web development
On Friendly Bit: 4 tier web development
I think of the 4 tiers as content, structure, presentation and behaviour. Almost the same. The content (or data) and the structure go together since, usually, there is (for example) a title that should be placed into a <h1> tag, chapters into <h2> etc. When it all looks readable without presentation then I add the design, using CSS. I don't like to add tags to accomodate design, I try to use only the tags necessary to the structure. The last layer (behaviour) should be unobtrusive, which basically means that is not mixed up with the structure but sits in an external file. I removed all inline Javascript calls after reading
separating behaviour and structure in 2004. The point of all this is just to build a site easy to update and apply changes to.