Initializr is here to kick-start the development of your new projects. It generates templates based on HTML5 Boilerplate by allowing you to choose which parts you want or don't want from it. A responsive template has also been added to start from a basic design instead of a blank page.
"H5BP" is here to handle the boring stuff you don't want to deal with. It provides a rock-solid default code that improves the cross-browser compatibility for HTML5 projects and handy tools to help their development.
You may have noticed that HTML5 Boilerplate also provides a builder on its main page, called "Boilerplate Custom". Actually this builder is Initializr. It uses the same backend, so feel free to use whichever you prefer, it's exactly the same!
Initializr functioning is pretty intuitive but it can help to read guides about it in your own language. Here are some which will help you using Initializr and understanding HTML5 Boilerplate, HTML5shiv or Modernizr, in French (official), English, Spanish, Russian, Croatian, Dutch and Portuguese. If you write an nice article in a missing language, I will put your link here!