On any timeline, as its namesake suggests, Hovercards are cards which appear when you hover over a username or avatar. The cards display additional information about the person and allow you to interact with them while staying within the context of your page.
One way we've found these cards to be useful is to find out more about retweeted people and follow them right there. You can also see more information with an expanded view of the card.
Hovercards will be rolled out in stages so not all of you will be seeing them right away.
[edited by: rogerd at 7:10 pm (utc) on Feb 4, 2010] [edit reason] fix typo [/edit]