Twitter made its name on the basis of brevity. You have 140 characters to get your point across, and that's it. This limit has also been in place for direct messages this whole time, but that changes in July. Twitter has posted an overview of the new DM API features so developers can get ready for longer messages.
The character limit will be raised from just 140 to a whopping 10,000. For all intents and purposes, that's unlimited. Anyone who's sending 10,000 characters in a Twitter DM needs to rethink life. Twitter has this change queued up and will flip the switch next month, but in the meantime developers need to make some small tweaks to make sure their apps are ready.
If an app hasn't been updated to support longer messages, it will still receive a truncated version of the DM with (probably) just 140 characters. Developers just have to add “full_text=true” to their query and they'll get the full length version. Sending longer messages will be supported automatically with no changes. Depending on an app's UI, developers might also need to make a few tweaks to the layout to support longer messages. Overall, it should be a painless transition.