WebAssembly is (currently) the same as asm.js, but with smaller file size and faster parse times. Asm.js is just a subset of JavaScript that can be optimized because it doesn't use certain features (like strings or garbage collection).
WebAssembly and asm.js are both intended to be compile targets - you don't write them by hand, but instead write your code in another language (c, Rust, etc.) and then complie to them.
The main benefits are 1) JavaScript is no longer the only language of the web, and 2) it's possible to get better performance than JS ever allowed for.
There is a lot of c code out there that probably isn't worth rewriting fron scratch in JS, but may well be worth recompiling to run as a web app.
Also, in the future, WebAssembly should get DOM access, optional garbage collection and other features that will allow it to be a compile targets for other languages such as Python and Ruby. So then you can use a single language for all of your development without that language being JavaScript.
WebAssembly and asm.js are both intended to be compile targets - you don't write them by hand, but instead write your code in another language (c, Rust, etc.) and then complie to them.
The main benefits are 1) JavaScript is no longer the only language of the web, and 2) it's possible to get better performance than JS ever allowed for.
There is a lot of c code out there that probably isn't worth rewriting fron scratch in JS, but may well be worth recompiling to run as a web app.
Also, in the future, WebAssembly should get DOM access, optional garbage collection and other features that will allow it to be a compile targets for other languages such as Python and Ruby. So then you can use a single language for all of your development without that language being JavaScript.