What is a responsive website and what is Bootstrap?
A responsive web site is one that is available on all devices.
You can have 1 web site that will respond to the size of the device display.
Therefore it is possible to have a different look on a mobile phone. Generally a desktop PC website will not look very nice or be very usable on a mobile phone or tablet. If your web site is responsive then it will adapt according to where it is being displayed.
In a world full of mobile devices it is very important that your web site is usable on smaller screen sizes.
Bootstrap is a freely downloadable framework originally developed by twitter. It is an attempt to standardise the way web sites are displayed across multiple devices.