What is Lightning
Salesforce Lightning is a platform which consists of the Lightning Aura Framework which is used to create single page responsive applications in salesforce. It consists of the following :
Lightning Experience is a single page user interface which is responsive and designed to improve productivity and speed.
Lightning Components standard and custom provide a flexible way to implement functionality in the lightning experience, communities and the salesforce mobile app (Salesforce1). These are of two types : aura and web components.
Lightning App Builder provides a way to create pages via point and click for admins. These pages are used as containers for our lightning aura/web components.
Experience Builder provides a way to develop communities with point and click using standard lightning aura/web components and templates.
Lightning Framework provides a way to develop custom standalone aura/web components which can be used in the lightning experience and the salesforce mobile app (Salesforce1).
Salesforce Lightning Design System (SLDS) provides a css library and best practices to implement the best user interface (UX) for the lightning experience.