1. Bootstrap
Direct quote from source: bootstrap/

2. Bootstrap is developed and maintained in GitHub and has a huge community of contributors. It is one of the top projects on GitHub. Now the version 4 of Bootstrap is expected to release soon and presently is in the first beta. There is one more beta version expected before the release candidates of the version 4.

3. What is Bootstrap Used For?
Bootstrap is used to create mobile-first, responsive websites quickly and easily. What it means is that you can easily create a website that scales itself to the size of the viewport of the device a person is using (responsiveness) and the website design is primarily intended for the mobile devices and then built up to suit the larger viewport devices (mobile-first).
Bootstrap consists of components that you can use to build your websites. These components have default styles which you can alter using Bootstrap’s contextual and utility classes or by using custom CSS that you can include in your markup. Bootstrap also has JavaScript components and uses jQuery plugins for components like tooltip and popovers. The beauty of Bootstrap is that if you don’t want to use JavaScript or jQuery in your project you just don’t have to include it. The choice is yours.
What is Bootstrap Used For?

4. There are many advantages to using Bootstrap
There are many advantages to using Bootstrap. First being it is open source so if you are starting with Bootstrap, you don’t have to worry about your wallet. And it is very easy to get started – all you need is a good understanding of HTML and CSS.
Once you get started you can develop your project in much less time than that you would take otherwise to build a responsive website. If you run into any issue while the development, the Bootstrap community is huge and you can find solutions to pretty much any issue, if not all. And if your design wants more out of Bootstrap than the styles provided by Bootstrap, it is easily customizable. Also, Bootstrap is compatible with all the modern browsers.
One other advantage of using Bootstrap is that you don’t have to start coding from scratch. The Bootstrap official website provides the code snippet for the basic HTML markup and for the various bootstrap components. Also, you can download a whole theme or a web page template so that you only need add your content and do some customizations. And you can also easily find code snippets, themes, templates, UI kits, plugins and add-ons for Bootstrap on the internet.
Why Use Bootstrap?

5. Where do I Get Bootstrap From?
You can download Bootstrap from their official website. You can either download Bootstrap containing only the CSS and JS file or download the entire source files. You can also include Bootstrap via CDN which is a good way to make your website load faster.
Where do I Get Bootstrap From?

6. How Do I Start Using Bootstrap?
After downloading Bootstrap, in the basic HTML markup, you need to include the Bootstrap CSS file in the head tags and Bootstrap JavaScript and jQuery just before the closing body tag. And that’s it. You are good to go.
Before beginning with Bootstrap, go through the documentation on the Bootstrap website. The documentation lists all the components in Bootstrap and example codes, there are a plenty of utility classes that can help create the layout very easily, the Bootstrap grid system and Flexbox are unavoidable while creating a website. The documentation has everything you need to know about Bootstrap.
When building your website you can make use of example code in the Bootstrap documentation. Also, there are plenty other resources that you can make use of online. Make sure you use them.
Some of the official resources for Bootstrap are:
Bootstrap Expo – showcases websites that are using Bootstrap
Bootstrap Themes – customizable official Bootstrap 4 Themes
Bootstrap Blog – official blog for Bootstrap
Bootstrap GitHub – Bootstrap is hosted, developed and maintained on Github
How Do I Start Using Bootstrap?

7. Are There Any Disadvantages?
There are some disadvantages to using Bootstrap. Since Bootstrap has a lot of reusable code for building you will simply recycle the code without actually understanding it.
When using Bootstrap you should be using some custom CSS for styling or you may end up looking like many other Bootstrap websites.
The file size of Bootstrap is large. This may affect the loading time of the website.
But these disadvantages can be overcome with customization. Bootstrap is easily customizable and can be used to create a stunning website. And about Bootstrap being bloated, the customize page in Bootstrap documentation is the solution. All you need to do is select only what you need and then recompile
Conclusion
Bootstrap is a powerful tool to create responsive, mobile first web applications .... It is the most popular front end development framework in the world.
BootstrapDash --- free and premium bootstrap admin dashboard template 
Are There Any Disadvantages?