Lightning Pages are single-page applications that replace existing Visualforce-based multi-page applications and their tabs. They also include Lighting Components for portions of their functionality. They are served from a Lightning server instead of a Visualforce server and can contain Visualforce wrapped in an I-frame. Additionally, there's the Salesforce Lightning Design System [SLDS] which is a flexi-page layout scheme with its own CSS markup. Further, Salesforce has "re-branded" its edition offerings and named them Lightning Editions for all the various packages.