As far as I understand bootstrap is a way to use the predefined classes. My question is where can I get all the classes which I should use. I mean the bootstrap documentation.
Be sure to check the version you’re using on the upper-right corner of the page.
Bootstrap is a framework to style your website. It contains lots of different components, like navigation, buttons, panels or many more. And yes, it is based on classes you have to add to your HTML in order to get the styles.
If you are getting started with Bootstrap, you can have a look into their (very good) documentation: http://getbootstrap.com/docs/4.0/getting-started/introduction/
Their documentation is separated into sections (see the menu on the left), each section contains the different styles. They explain thoroughly how to use the styles and what they do.
If you’d like a video tutorial, I can recommend this one on YouTube: https://www.youtube.com/playlist?list=PL4cUxeGkcC9jE_cGvLLC60C_PeF_24pvv
Be sure to use tutorials or resources for Bootstrap 4, as this is the latest version.