What is the different between require, require.resolve, webpack.ProvidePlugin and webpack.DefinePlugin

Question

I'm doing a project using webpack, angularjs. In the webpack.config.js, I've seen sometimes they use:

In app.js

require('angular')

In webpack.config.js

new webpack.DefinePlugin({
    'require.specified': 'require.resolve'
}

In webpack.config.js

new webpack.ProvidePlugin({
    Promise: 'bluebird'
}

In webpack.config.js

{
                test: require.resolve('rxjs/bundles/Rx'),
                use:[
                    {
                        loader: 'expose-loader',
                        options: 'Rx'
                    }
                ]
            }

I don't understand what are their differences. When should I use require, require.resolve, webpack.DefinePlugin, webpack.ProvidePlugin

Can anyone help me please ?

Thank you