webpack has a rich plugin interface. Most of the features are internal plugins using this interface. This makes webpack very flexible.
webpack uses async I/O and has multiple caching levels. This makes webpack fast and incredibly fast on incremental compilation.
webpack allows you to split your codebase into chunks. Chunks are loaded on demand. This reduces initial loading time.
webpack's main target is the web, but it also supports generating bundles for WebWorkers and Node.js.