The hCard format is Semantic HTML created through the Micoformats process, and based on the vCard format, with additions and modifications made based on user feedback.
Like with all Semantic HTML, including Microformats, hCards are used to make things easier for machines to understand what you are writing about in HTML. With hCards, it makes contact info easier to understand, for machines.
Say we wanted to create an e-mail signature like...
Joe Blow, B.Sc.
Then we could mark this up with an hCard as follows...
<span class="fn n"><span class="given-name">Joe</span> <span class="family-name">Blow</span>, <span class="honorific-suffix">B.Sc.</span></span>
e-mail: <span class="email">firstname.lastname@example.org</span>
phone: <span class="tel">604-555-5555</span>
blog: <span class="url">http://blow.dom/</span>
Here is another example, that cleverly compacts alot of hCard semantics into a tight space...
<a class="vcard fn n url" href="http://blow.dom/">Joe Blow</a>
hCard Class Names
HTML address Element