WeChat Mini Programs are small applications that run inside WeChat, the messaging app made by Tencent. Tencent describes the platform as a development framework with components and APIs that lets developers create services in WeChat with an experience similar to native apps. Users open and share them inside WeChat, often by scanning a Mini Program code, a kind of QR code.
The main language is JavaScript, with WeChat's own markup (WXML) and style sheets (WXSS). Unlike a web page, a Mini Program has a separate logic layer and rendering layer that run in different threads, so browser DOM APIs, libraries such as jQuery and some npm packages do not work. A developer applies for a Mini Program account, builds and previews the project in the WeChat developer tools, uploads the code, and submits it for Tencent's review; after approval it is released to all users or in stages. APIs give access to WeChat features such as payment, user information and device hardware.
It is a platform run by Tencent. The downloaded documentation pages do not state fees for registering or publishing.