I haven't touched wasmer specifically, but as a wasm noob who's been playing with dart2wasm/emscripten recently, I'll try and explain as best I can. The ecosystem is a bit messy, so it can be difficult to wrap your head around. If I've got anything wrong, someone please jump in and correct.
wasm is just a bytecode assembly format - you can compile something like "int a = 1; int b = 2; return a+b", but to actually do anything interesting you usually need some kind of runtime environment where you can spawn threads, access a filesystem, print to stdout, etc etc. WASI, for example, is an interface for accessing system libraries via wasm; runtimes like wasmer/wasmtime (for native) or v8 (Chromium) will expose their own implementations of WASI. WASIX is wasmer's own extension for WASI (a point of some contention in the wasm community).
Running wasm in a browser, though, still requires some manual setup with Javascript, along with crossing your fingers that the browser runtime actually supports the system calls for whatever you've compiled.
I guess what wasmer has created here is a Javascript SDK so you can:
1) easily download wasm binaries from a central registry
2) ensure those binaries actually run as intended in a browser environment
3) avoid the manual setup Javascript code currently needed to run wasm modules.
emscripten is a (non-WASI) runtime + compiler so this isn't an exact alternative to emscripten (I guess in theory you could compile something via emscripten, publish to the wasmer registry, then consume solely in Javascript). It does, however, replace the manual JS wiring you need to do to run emscripten-compiled binaries in the browser.
wasm is just a bytecode assembly format - you can compile something like "int a = 1; int b = 2; return a+b", but to actually do anything interesting you usually need some kind of runtime environment where you can spawn threads, access a filesystem, print to stdout, etc etc. WASI, for example, is an interface for accessing system libraries via wasm; runtimes like wasmer/wasmtime (for native) or v8 (Chromium) will expose their own implementations of WASI. WASIX is wasmer's own extension for WASI (a point of some contention in the wasm community).
Running wasm in a browser, though, still requires some manual setup with Javascript, along with crossing your fingers that the browser runtime actually supports the system calls for whatever you've compiled.
I guess what wasmer has created here is a Javascript SDK so you can:
1) easily download wasm binaries from a central registry
2) ensure those binaries actually run as intended in a browser environment
3) avoid the manual setup Javascript code currently needed to run wasm modules.
emscripten is a (non-WASI) runtime + compiler so this isn't an exact alternative to emscripten (I guess in theory you could compile something via emscripten, publish to the wasmer registry, then consume solely in Javascript). It does, however, replace the manual JS wiring you need to do to run emscripten-compiled binaries in the browser.