Besides the structural and grouping elements we've looked at, HTML5 also revises a few tags that used to be referred to as inline elements. The HTML5 specification now refers to these tags as text-level semantics (http://www.w3.org/TR/html5/text-level-semantics.html#text-level-semantics). Let's take a look at a few common examples.
Historically, the <b> element meant "make this bold" (http://www.w3.org/TR/html4/present/graphics.html#edef-B). This was from back in the day when stylistic choices were part of the markup. However, you can now officially use it merely as a styling hook in CSS as the HTML5 specification now declares that <b> is:
"The b element represents a span of text to which attention is being drawn for utilitarian purposes without conveying any extra importance and with no implication of an alternate voice or mood, such as key words in a document abstract, product names in a review, actionable words in interactive text-driven software, or an article lede."
Although no specific meaning is now attached to it, as it's text level, it's not intended to be used to surround large groups of markup, use a div for that. You should also be aware that because it was historically used to bold text, you'll typically have to reset the font-weight in CSS if you want content within a <b> tag to not appear bold.
OK, hands up, I've often used <em> merely as a styling hook too. I need to mend my ways, as in HTML5:
The em element represents stress emphasis of its contents.
Therefore, unless you actually want the enclosed contents to be emphasized, consider using a <b> tag or, where relevant, an <i> tag instead.
The HTML5 specification describes the <i> as:
"...a span of text in an alternate voice or mood, or otherwise offset from the normal prose in a manner indicating a different quality of text."
Suffice it to say, it's not to be used to merely italicize something. For example, we could use it to markup the odd name in this line of text:
<p>However, discussion on the hgroup element is now frustraneous as it's now gone the way of the <i>Raphus cucullatus</i>.</p>
There are plenty of other text-level semantic tags in HTML5. For the full run down, take a look at the relevant section of the specification at the following URL:
http://www.w3.org/TR/html5/text-level-semantics.html#text-level-semantics