Bootstrap is a very useful front-end framework for faster and easier web development. You won't have to worry about having a decent user interface when you use it.
I personally love this framework because I'm not into designing web app user interfaces, it solves a pain in me. That is why I made this Bootstrap tutorial for beginners.
We are using Bootstrap for web application tutorials as well. One example is our PHP, MySQL and OOP CRUD Tutorial – Step By Step Guide!
Here are some good looking websites or web apps built with the help of bootstrap: http://expo.getbootstrap.com
Just a little history, Bootstrap is created in twitter, it was called Twitter Bootstrap to streamline their development. Thanks to these people, the framework is still in active development.