Bootstrap is a CSS framework that makes it easier to create website and web application user interfaces. Bootstrap is especially useful as a base layer of CSS to build sites with responsive web design.
Bootstrap 3 was out for almost five years before Bootstrap 4 was finally released at the start of 2018. When learning Bootstrap you will likely come across many tutorials that cover Bootstrap 3 but not version 4.
If you are completely new to Bootstrap and CSS frameworks in general then I recommend learning Bootstrap 4. If you have already been working with Bootstrap 3 then there is no major rush to upgrade to the latest version. Bootstrap 4 is more complicated than version 3 because it has a lot more features so the learning curve is a bit steeper.
Full Stack Python is actually built with an early version of Bootstrap 3. However, this site is so heavily customized with my own CSS that I likely will never upgrade to Bootstrap 4 because there are no new features that I feel will be useful in my specific situation.
Getting Started with Bootstrap provides a walkthrough of a sample starter template so you can understand the CSS classes that Bootstrap uses. There is also a follow up post that shows how to create an effective sales page with Bootstrap 3.
What is Bootstrap and how do I use it? is an awesome tutorial that goes through many of the main Bootstrap elements such as icons, navigation bars and "jumbotron"-style webpages.
Bootstrap 4: What's New? shows a slew of comparisons for how user interface elements have changed design over the past several Bootstrap major version iterations.
How to use Bootstrap 4 forms with Django shows how to combine the popular django-crispy-forms library with Django to create and obtain user data through web forms that are styled with Bootstrap 4 CSS.