Lightning has really expanded since it was originally introduced at Dreamforce 2014. Originally, Lightning was introduced as a couple of different products that were all sort of related because they had the same incredibly new looking UI.
Lightning Process Builder was designed to allow admins to intelligently do a lot of things that previously required developers to do in apex before and after triggers.
How does Lightning Work?
Resources for Learning Salesforce Lightning
There’s quite a few resources available for learning the different parts of Salesforce Lightning. Trailhead is a really resource for learning it.
Salesforce has some pretty good trails on Trailhead for developers and administrators. Get Started with Lightning Experience is the resource for Administrators. Develop for Lightning Experience is a really good resource for Developers.