Questions related to Lightning CSS Framework called Salesforce Lightning Design System or SLDS for short. Specifically questions on how to use, getting certain features to work, and questions related to any of the suggested code or use cases on the Lightning Design System Website: https://www.lightningdesignsystem.com/ Lightning Experience UI, Lightning Components, Visualforce, and Lightning App Builder are all different but related tags.
Salesforce Lightning Design System (SLDS) is a CSS framework designed by Salesforce to match the Lightning user interface. This CSS Framework is documented on its own microsite, has its own github repo, and has a number of Trailhead modules and projects to help developers learn to use it.
The creation of a CSS framework to mirror the look and feel of Salesforce gives developers a new option. Where before a developer might have had to reverse-engineer the css classes from the Salesforce UI, or directly reference Salesforce css classes in their raw markup, SLDS hands over specific CSS classes, structures, and code best practices to enable a developer to create a UI that looks just like Salesforce, without having to build the look and feel from the ground up.