According to webpack documentation @ https://webpack.github.io/docs/tutorials/getting-started/#watch-mode
When using watch mode, webpack installs file watchers to all files,
which were used in the compilation process. If any change is detected,
it’ll run the compilation again. When caching is enabled, webpack
keeps each module in memory and will reuse it if it isn’t changed.
So, basically, the difference between running webpack and webpack --watch is that in using --watch, your CLI will hang after the compilation process waiting for any code changes in your files and if there is a change, then it will recompile and wait again. You should be aware that if you are using webpack-dev-server, then you do not need to use this option because webpack-dev-server uses webpack’s watch mode by default according to its documentation:
The dev server uses webpack’s watch mode. It also prevents webpack
from emitting the resulting files to disk. Instead it keeps and serves
the resulting files from memory.
So, what is webpack-dev-server --hot? Basically, this adds the HotModuleReplacementPlugin to the webpack configuration, which will essentially allow you to only reload the component that is changed instead of doing a full page refresh! Pretty darn useful when you are working with states! According to the documentation:
Each mode also supports Hot Module Replacement in which the bundle is
notified that a change happened instead of a full page reload. A Hot
Module Replacement runtime could then load the updated modules and
inject them into the running app.
More information on what it is and how to used it here: https://webpack.github.io/docs/webpack-dev-server.html#hot-module-replacement
I hope this helps in understanding webpack a bit more!