Lightning Experience is the new interface for the user’s desktop interaction with Salesforce. It was designed to resemble the Salesforce1 mobile app for smartphones. Nearly all of the original functionality is identical to the original Salesforce Classic interface, Lightning has rearranged much of the layout, and so it requires a bit of acclimating. And while the new interface has been officially released already, Salesforce is still finalizing much of the essential functionality.
Here’s a quick video demonstration: https://youtu.be/lZK5a4hC1uk
Here’s more information: