In HTML, <div> is a layout and design tool. When you surround a block of text, or a block of text and images, in <div> tags (HTML), you can use id or class (CSS) to position or change other aspects (such as the background color or width) of only that block.
Let's look at the elements of CSS you really need to know well:
There are five of these:
You really need to specify ALL FIVE in your CSS. If you don't, users are likely to see some ugly color effects. Your links might even be invisible!
If you don't feel like specifying all of them, at least spec a and a:hover. But be aware that a alone will also affect name anchors: <a name> </a> in HTML, which you may or may not be using. (See this page for more information about name anchors -- "the name Attribute" -- and links in general.)
Many Web designers use a "reset" stylesheet. The most commonly used one comes from Eric Meyer. You can download it (and also learn all about what it does) at his Web site.
Between <div> and <span> (see an explanation).
Between display:block; and display:inline; (see an explanation).
Between id and class (see an explanation).
I wrote a blog post about this in May 2010: Tips for HTML5, part 6: A look at CSS3. Summary: The CSS we have been using stays the same, and some cool new capabilities are being added.
Go to w3schools.com for CSS tutorials.
CSS3 Quick Reference Guide (PDF, 5 pages, 124 KB)
See my Delicious bookmarks tagged "CSS."