You don't use a specific HTML tag, but rather semantic annotation. You have two options: Microdata and RDFa.
Both have similar capabilities and are understood by search engine crawlers. That said Google is a supported of Microdata.
For ready-to-use examples you can refer to schema.org website. Each term definition show usage with both serializations. Your HTML could be:
Microdata
<p itemscope itemtype="http://schema.org/Person">
The ultimate measure of a man is not where he stands in moments of
<span itemprop="description">comfort and convenience</span>,
but where he stands at times of challenge and controversy.
</p>
RDFa
<p vocab="http://schema.org/" typeof="Person">
The ultimate measure of a man is not where he stands in moments of
<span property="description">comfort and convenience</span>,
but where he stands at times of challenge and controversy.
</p>
As you can see I simply use <span>
to encapsulate annotated text. Other that that RDFa and Microdata aren't that different.
Also, do have a look at RDF Translator app. It will help you fiddle with your annotations.