Twitter's 140 character rule still exists, but the company has been taking some steps to mitigate it somewhat recently. The newest such move comes today, when it's announced that usernames aren't counted towards that 140 character tweet limit anymore.
What this means in practice is that you will get slightly more space to express your thoughts when you're addressing other individual users of the service, and a lot more space in group conversations. From now on who you're replying to will appear above the tweet and not within the tweet text, so you have more characters for the actual conversation.
If you tap on "Replying to..." you can easily see and control who's part of a conversation. Perhaps most importantly, when you read through a conversation, you'll actually see what people are saying instead of first having to read a bunch of usernames at the start of each tweet. This obviously makes it easier to follow a conversation.
In its tests of this new experience, Twitter has found that people were more engaged with conversations. The update is now rolling out on the website, as well as in the mobile apps for Android and iOS. Make sure you have the latest version of Twitter if you want to check this out.