This article explains the high level concepts behind WebAssembly including its goals, the problems it solves, and how it runs inside a web browser's rendering engine.
The Bytecode Alliance is an open source community dedicated to creating secure new software foundations using WebAssembly and the WebAssembly System Interface (WASI).
WebAssembly is an assembly language for a conceptual machine but to work outside of the browser it also needs a system interface for a conceptual operating system. Lin Clark's introduction to WASI is the best summary of its goals and features.
Wasmtime is a standalone runtime for WebAssembly that works outside of web browsers. It fits on tiny chips and effectively scales up up on servers. Wasmtime is designed to be embedded in many different applications.
This proposal adds a new set of interface types to WebAssembly that describe high-level values like strings, sequences, records and variants. This encourages better interoperation between WebAssembly modules and with external application programming interfaces like Web APIs.