You’ll see a popover box with their:
a link to add them to circles (or showing what circles you have them in already)
their tag line and employment or location
Hover cards can be found in over twenty
places across various Google properties
(including Gmail and YouTube) as detailed in
Stephan Hovnanian’s excellent article
Just imagine, 24 new opportunities to get
in front of potential clients!
The Google+ Hover card will trigger on:
All posts and status updates
The +1 activity on any comment
A +mention in a post (or comment)
Your posts when someone is browsing the related hash
tags on a post
and many, many more!