It's a HTML/CSS/Javascript framework. It's basically a collection of pre-written code which you can tie into e.g. button styling, form styling, responsive CSS etc.

The main advantage is that they have already done all the, sometimes tedious work for you. So if you use the responsive CSS, it will be responsive on all devices and work in most browsers straight away, for example.

I use it to quickly mockup working examples of ideas.

Bootstrap is well know in responsive design now a days . Its use for responsive design. Actually bootstrap css is normal css. They make a file of it . You can download free from internet.

can we create a responsive website without using bootstrap????

Off course. Bootstrap is just there to make things easier, it's good for rapid prototyping, fast development, but you can't always rely on it. I prefer to use my own solutions, however, I think bootstrap is very useful!

I'll add in that Twitter Bootstrap in particular is a very nice version of Bootstrap

Hello, I would like to inform you that there are many other CSS frameworks other than twitter bootstrap, if you are new to CSS frameworks then you could start with skeleton.css which is another CSS frameworks and a lot less CSS heavy and easier to understand then bootstrap.

My website uses skeleton. Take a look. Website Design Surrey