From the Electronic Frontier Foundation article:
"TextSecure is a messaging app that allows users to send confidential messages with their mobile phone using end-to-end encryption. The app works over a Wi-Fi or data connection, but can also be used to send SMS if such a connection is unavailable.
TextSecure can also be used to send SMS to non-TextSecure users, however, those messages will not be encrypted in transit. Encrypted text messages are marked with a small lock. Messages in blue are sent over a data connection, while those in green have been sent over SMS. Both sides have to be using TextSecure for the messages to be secure while they travel over the Internet. You can set up a passphrase that will encrypt these conversations when they are stored on your local phone. This means that they are better protected against being read if your phone is seized or compromised.
TextSecure is available in more than 30 languages. You can change the language of the app by selecting “Settings” then “Language.”"