The Internet is full of Bootstrap resources; here are a few:
Here are a few more resources for Bootstrap, but there are many more. You can surf through these sites and gain some more knowledge about the working and intricacies of Bootstrap:
As of now, Bootstrap version 4 alpha has been launched. All the JavaScript plugins have been rewritten in ECMAScript6, Completely shifted from LESS to SASS and IE8 support has been dropped.
Here is official blog post: http://blog.getbootstrap.com/2015/08/19/bootstrap-4-alpha/
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