Disclaimer: The following are glossaries I've written to simplify sharing what the Signal application is all about with friends or others. I am not associated with Signal and/or Open Whisper Systems in any way.
Signal is a free and open source software application for Android, iOS, and Desktop that employs end-to-end encryption (E2EE), allowing users to send end-to-end encrypted group, text, picture, video messages or other attachments, and have encrypted phone conversations between Signal users. Although Signal uses telephone numbers as contacts, encrypted calls and messages use your data connection so both parties to the conversation must have internet access on their mobile devices.
See the official Open Whisper Systems website (WhisperSystems.org) for more information on Signal. Note that Signal.org is the same website as the official Open Whisper Systems website.
Source code for Signal Chat is available on the Open Whisper Systems Github Organization page (github.com/whispersystems).
App for Android devices is available on Google Play Store.
App for iOS devices is available on iTunes App Store.
A desktop application is available as a Chrome plugin on Chrome Web Store, you can read more about the Chrome plugin in this blog post on WhisperSystems.org or go straight to Chrome Web Store to get the plugin.
A couple of notes for the Signal Chrome plugin that are good to keep in mind:
- You need to create an account using either the Android or iOS app before you can use the Signal Chrome plugin.
- You can use this Chrome plugin either with the Google Chrome browser or the open-source Chromium browser.
- The Signal Chrome plugin only enables you to read and send text messages. Sending sms/mms and voice calls are currently only possible using the Android or iOS app.
Open Whisper Systems, the makers of Signal, use infrastructure of other companies to send its users alerts when they receive new messages. On Android they use Google infrastructure, and on iPhone they use Apple infrastructure. This means information on who is receiving messages and when they were received may leak to these companies.
Also keep in mind if you decide to use the Chrome plugin with a Google Chrome browser where you're logged into your user account, your text messages in Signal are accessible on all desktop devices where you're logged into Google Chrome. Edit: this was an assumption error on my part, the Signal Chrome plugin gets installed on all desktop devices where you're logged into Chrome, but you'll need to go through the usual account connection steps on each desktop you wish to use.