Signal is a cross-platform centralized encrypted messaging service developed by the Signal Technology Foundation and Signal Messenger LLC. It uses the Internet to send one-to-one and group messages, which can include files, voice notes, images and videos. It can also be used to make one-to-one and group voice and video calls, and the Android version can optionally function as an SMS app.
Signal uses standard cellular telephone numbers as identifiers and secures all communications to other Signal users with end-to-end encryption. The apps include mechanisms by which users can independently verify the identity of their contacts and the integrity of the data channel.
Signal’s software is free and open-source. Its clients are published under the GPLv3 license, while the server code is published under the AGPLv3. The official Android app generally uses the proprietary Google Play Services (installed on most Android devices), though it is designed to still work without them installed. Signal also has an official client app for iOS and desktop apps for Windows, MacOS and Linux.
The non-profit Signal Foundation was launched in February 2018 with initial funding of $50 million from Brian Acton. As of January 2021, Signal had more than 105 million total downloads, and the app had approximately 40 million monthly active users. Signal has been installed on more than 50 million Android devices. (Source)
Threema (not free)
In addition to text messaging, users can make voice and video calls, send multimedia, locations, voice messages, and files. A web app version, Threema Web, can be used on desktop devices, but only as long as the phone with the Threema installation of the user is online.
Threema is developed by the Swiss company Threema GmbH. The servers are located in Switzerland and the development is based in Pfäffikon SZ. As of January 2021, Threema had 9 million users and the business version, Threema Work, was used by 5,000 companies and organizations. (Source)