Which editor is used to create this code?
<span class="sprocket-pagination-title">Content delivery via a flexible<span class="hidden-tablet sprocket-pagination-title">, dynamic and configurable</span> framework</span>
When creating content for an article or module, then inside Joomla you will use some variety of HTML editor. That's where this kind of code would be introduced.
Where do I learn to use these class tags?
1) Google is your friend