Hovercards will be a new way to access profile information for people without having to actually visit their Twitter profile.
“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.”
By hovering over the username in your timeline you get profile information, location, follow status and an option menu to select follow, block, mention, or report abuse.
Looking at the screen capture on the Twitter blog it looks like this feature will be useful, but like everything they release, this feature isn’t available for everyone just yet, it’s being phased out over time.
Keep in mind, that this will only be active on twitter.com and not a standalone client application like Tweetdeck, Seesmic or HootSuite.