Is no one able to answer this question?

hash is calculated for a build, chunkhash is calculated for chunk (entry file), contenthash is special hash generated in ExtractTextPlugin and is calculated by extracted content, not by whole chunk content

See this article https://medium.com/@okonetchnikov/long-term-caching-of-static-assets-with-webpack-1ecb139adb95#.f9aon6s3b

The question is good (I had the same)... It is mostly about Webpack, but indeed contenthash is specific to this plugin.
Additional references: https://webpack.github.io/docs/code-splitting.html explains chunks (not very clear...), and http://survivejs.com/webpack/building-with-webpack/splitting-bundles/ (and other chapters) can bring more information.
@ahmed1490 if your question is answered, you should close this issue...

It's old, but If anyone came here and still confused, see my answer on Stackoverflow with examples:
https://stackoverflow.com/a/52786672/5263363