Secure SMS is a fully functional messaging app, able to read, write, send, and receive plain and encrypted text messages. It uses the industry standard OpenPGP protocol with 1024-bit RSA and 128-bit AES keys.
You can lock your mailbox so no one can read your stored messages. If you select this option (via the Preferences menu item), SecureSMS will run a background service which watches your Inbox and Sent folders. All plain text messages will be encrypted; existing stored messages will be converted immediately, and all subsequent messages will be converted upon arrival.
If your friends have SecureSMS, you can share your public keys with each other (via the Share Key menu item). Then you will be able to exchange secure messages which cannot be read, even if they are intercepted in transit.