What is SLDS in Salesforce Lightning? What is the use of this?

Hi,

Lightning Design System is quite a mouthful, so we’re referring to it here as the “Design System”.

You might have used other similar design systems, such as Twitter Bootstrap or Foundation for building websites. The key benefits of this Design System are:

It’s tailored for building Salesforce apps. Using the Design System markup and CSS framework results in UIs that reflect the Salesforce Lightning look and feel. The focus on building applications is an important one to keep in mind. The Design System does not over-enforce defaults such as padding and margins, in contrast to some other frameworks which are focused on building marketing pages. The Design System lets you easily specify the exact layouts you require, whilst conforming to the new Lightning UI.
It’s continuously updated. As long as you’re using the latest version of the Design System, your pages are always up to date with Salesforce UI changes.
Accessibility is baked in to the CSS framework behind the components.
The CSS is fully namespaced with the slds- prefix and scoped with the slds-scope class to avoid CSS conflicts.
The Design System bundles four types of resources to help you build applications.

CSS framework — Defines the UI components, such as page headers, labels, and form elements, a grid layout system, and a single-purpose helper classes, to assist with spacing, sizing, and other visual adjustments.
Icons — Includes PNG and SVG (both individual and spritemap) versions of our action, custom, doctype, standard, and utility icons.
Font — Typography is at the core of our product. We’ve designed the new Salesforce Sans font from the ground up to give our product a distinct visual voice and personality, and we’re making it available to you as part of the Design System.
Design Tokens — These design variables allow you to tailor aspects of the visual design to match your brand. Customizable variables include colors, fonts, spacing, and sizing.

Thanks