Bootstrap is essentially a front-end Framework that makes designing webpages and the general development easier. Bootstrap uses CSS, however it's different to writing your own CSS as it's all pre done so you just add a class to a HTML element and then it will use that CSS. So for example adding the class "img-responsive" to will use the pre made img-responsive class which is in the external Bootstrap CSS file.

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Bootstrap is great because it allows you to easily create a responsive, cross browser compatible website really easily. For example making the site responsive without Bootstrap would require you using Media Queries etc, whereas with Bootstrap you can just give your divs class names to make it responsive.

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Essentially Bootstrap uses CSS. But it's different to writing your own CSS as it's all pre made for you and you just need to import the external CSS file made by the people at Bootstrap, and you just need to know which classes to use etc.