There was some consternation last year as rumors swirled that Twitter was set to drop its long-standing 140-character limit. The company ended up keeping the limit, but it's been working to make sure you can actually use all 140-characters. For example, Twitter decreed a while back that photos and videos no longer count against the limit. Now, it's doing the same for usernames. Finally.
To understand why this is a big deal, consider the following scenario: You reply to someone on Twitter, then a different person jumps in with a reply, and another person replies after that. Through no fault of your own, you'd now have to reply to three usernames if you want to continue including everyone in the conversation. That could easily take up half the 140 character limit, but those don't count at all anymore.
Twitter will only count the text you add to the tweet in the 140-character limit. The official app will list the @names above the tweet for clarity, and that's where you can control who's part of the conversation. This update is rolling out right now to the web and official apps. Presumably third-party apps will be able to implement this soon, but the way @names are displayed will be up to the developer.