Frameworks
A framework manages the layout of your web app or site to look good on different size screens.
Twitter and Google have both developed and released for free a library of code with two of the best and most popular frameworks on the web.
Bootstrap
Originally developed by Twitter and now available as open source code 'Bootstrap' is the most popular framework today.
thumb_up Pros - Sophisticated framework with endless customisation options, being the most popular framework there are lots of ready made themes and big online support.
thumb_down Cons - Larger file size so page speed will be slower, around ~64K with javascript ( jQuery required) or ~31K without.
Material Design Lite
The Material Design concept was developed by Google for apps and websites to make screen experience more consistent. Developers can use the Material Design Lite (MDL) code library for free to create a clean and fast web experience similar to using an app.
thumb_up Pros - Smaller file size with vanilla javascript and less code, around ~27K, much faster page speed and an improved page rank on Google.
thumb_down Cons - Less customisation as it was designed to follow Material Design principles for apps and mobile sites.