id is used for one type of thing, for example

HTML: Select all

<div id="wrapper">Content</div>

if you are defining the same tag in different ways then you use class

for example your navigation might be

HTML: Select all

<ul class="navigation>
</ul>

and you submenu might also be an <ul> tag, but this time it is

HTML: Select all

<ul class="sub-menu>
</ul>

Hope this isn't too confusing

Craig

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