There's a bit of mythology around the <u> tag. Like <b> and <i>, it has not been officially deprecated, because there are times when you want text to be underlined and that is the most semantically appropriate tag to use. However, whereas <strong> typically gives bold text and <em> typically gives italic text, and these carry more semantic meaning than <b> and <i>, there is no real equivalent for <u>.
And why is that? Because it's recommended that in general, you avoid underlining text on the web ... because underlined text looks like a link. Go on, hands up, how many of you were thinking about clicking on that? The convention that underlined text signifies a link is completely hard-wired into surfers, so if you have underlined text that isn't a link, it's going to be confusing for them and will make it harder for them to tell what is clickable and what isn't.
Sure, there will be times when underlining is what you need, such as when you are working to a particular legal or house style guide, or making a facsimile replica of a printed document, and in those cases it's fine to use <u> if there's no other handy semantic element to hang
text-decoration:underline; on ... but apart from that very small set of examples, you shouldn't be underlining anything except links, and with links there's no need to use <u>!