As you've described, Webpack is a module bundler, it bundles various module formats primarily so they can be run in a browser. It offers both a CLI and Node API.
Webpack Dev Middleware is middleware which can be mounted in an express server to serve the latest compilation of your bundle during development. This uses webpack's Node API in watch mode and instead of outputting to the file system it outputs to memory.
For comparison, you might use something like express.static instead of this middleware in production.
Webpack Dev Server is itself an express server which uses webpack-dev-middleware to serve the latest bundle and additionally handles hot module replacement (HMR) requests for live module updates in the client.
Webpack Hot Middleware is an alternative to webpack-dev-server but instead of starting a server itself it allows you to mount it in an existing / custom express server alongside webpack-dev-middleware.
Just to confuse things even more, there's also Webpack Hot Server Middleware which is designed to be used alongside webpack-dev-middleware and webpack-hot-middleware to handle hot module replacement of server rendered apps.
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webpack hot middleware是给webpack-dev-middleware用的，进一步增强服务器功能，使得服务器支持热更新
总结就是：webpack-dev-server包含webpack-dev-middleware和webpack-hot-middleware，是一个可以独立使用的开发服务器；webpack-dev-middleware是一个可以和express配合的开发服务器中间件；express + webpack-dev-middleware+ webpack-hot-middleware （webpack-dev-server就是它们集合实现的一种）可以组成一个支持热更新的静态资源开发服务器。