This is basically done with Meta tags
of the page you're targeting. Google Plus (and Facebook for example) fetch this metadata from the page you're linking to, parse it and show what they want to show.
It's not an actual iframe
or embed
.
Take a look at The Open Graph protocol: http://ogp.me/
For example if you look at the DOM of the OGP page you'll find some meta tags like:
<meta property="og:description" content="The Open Graph protocol enables any web page to become a rich object in a social graph.">
Now if you're paste that link (http://ogp.me/) into Facebook, that's exactly what will be displayed there. Google Plus doesn't fetch this description for example, it just takes og:image
and og:title
.
If you want to know how to grab these information of the specified page you should search for questions about that directly, for example here: How can I retrieve og/meta attributes of a resource?
Hope it helps.