WhatsApp is a messaging and calling app made by WhatsApp LLC. It runs on phones, desktop computers and tablets, and in a web browser as WhatsApp Web. People send text and voice messages, stickers, GIFs and files, and make voice and video calls.
Personal messages and calls use end-to-end encryption: they are encrypted on the sender's device and decrypted only on the receiver's, so not even WhatsApp can read them. A group can hold up to 1,000 people on Android and iPhone; admins choose who can add and remove members, and members can send polls, create events and start group calls. For companies there are two products. The WhatsApp Business App is for small businesses that answer customers personally. The WhatsApp Business Platform gives medium and large businesses programmatic access, through an API, to send marketing, authentication, utility and service messages at scale.
The app is free to use, apart from mobile data charges. The Business Platform has its own price list.