A basic tooltip on a link for instance would just be <a href="http://somesite.xxx//" title="If you can read this, you're hovering over the link!">Link</a>.
You'd probably have to use scripts if you want anything fancier. Sorry I'm not an expert on this.
I am looking for the code to use the FFXIAH Tooltips (the ones that show the items stats when you hover over them). You usually just have to add something into the header code, then just use a regular link to the item page here.