Q: Which HTML5 tags to use to wrap definitions and examples in a dictionary? What is the best way to HTML5ify this?
follow: Go or come after (a person or thing proceeding ahead); move or travel behind.

"She went back into the house, and Ben followed her."

This?
<article>
<dfn>follow</dfn>: Go or come after (a person or thing proceeding ahead); move or travel behind.
<q>"She went back into the house, and Ben followed her."</q>
</article>


A: Unless your definition is more extensive like an encyclopedia article (example, wiki), I would not call it an article since the <article> tag:
Represents a section of a page that consists of a composition that forms an     
independent part of a document, page, or site. This could be a forum post, 
a magazine or newspaper article, a Web log entry, a user-submitted comment, 
or any other independent item of content.

For a series of short definitions that you might have in an appendix of a book or white paper, I would suggest using a data definition list: <dl> width <dt> and <dd> to mark-up the term and definition respectively.
Reference: http://html5doctor.com
There is much discussion about how best to use HTML5 tags, makes for interesting reading.
How I Might Style This
If I were making a simple definition list for a glossary, I might start with:
<dl>
   <dt><dfn>follow</dfn>:</dt>
    <dd>
        <p>Go or come after (a person or thing proceeding ahead); move or travel
            behind.</p>
        <q>She went back into the house, and Ben followed her.</q>
    </dd>
</dl>

and apply some styling as follows:
dl {
    border: 1px solid #989898;
    border-radius: 10px;
    padding: 1.00em;
}
dt {
    margin-left: 1.00em;
}
dfn {
    font-weight: bold;
    font-size: 1.25em;
}
dd {
    margin-left: 2.00em;
    margin-bottom: 3.00em;
}
dd p {
    margin: 0 0 0.50em 0;
}
dd q {
    display: block;
    margin: 0 0 0.50em 0;
    font-style: italic;
}

This is just an illustration of how much flexibility and control you have over the various elements in the layout.
Fiddle Reference: http://jsfiddle.net/audetwebdesign/H6593/ 

A: HTML5 elements aren't needed, it's a case for the good old dl, dt and dd.
e.g.
<dl>
    <dt>Thing one</dt>
    <dd>
        <p>This defines thing one.</p>
    </dd>

    <dt>Thing two</dt>
    <dd>
        <p>This defines thing two.</p>
    </dd>
</dl>

dl is a definition list, dt is a definition term, and dd is the definition description.
