Mark Benioff, the CEO of Salesforce, started out at Oracle. Larry Ellison was his mentor. I think he was even at Larry Ellison's (3rd?) wedding. He left Oracle to start Salesforce.
Salesforce is a software layer on top of databases, often called CRM. It's the answer for "how do you sell a relational database to a guy that just needs a rolodex?" Salesforce actually uses Oracle databases to power their infrastructure, to this day.
Over the past 10 years, Salesforce has built out its own platform that basically turns Salesforce into a web-based version of visual fox pro. Which sounds bad, but consider that many "Salesforce.com administrators" are NOT programmers. Then to be taken more seriously as a custom development platform, to be able to add more advanced logic, they built their own scripting language (Apex) and bought Heroku.
So it started as (pretty much) a fancy rolodex but has evolved into a platform through which you can develop certain types of software and workflows that would be difficult with alternatives. "Difficult with alternatives" becomes less true every day (which is why I got away from pitching businesses on custom development with Salesforce and into pitching businesses on custom development with python and purely open solutions.) Despite having legitimate competition from open source, their sales and marketing has done a very good job at convincing their users that those don't exist, or that Salesforce invented them (cough heroku).
Maybe you've heard of Oracle or SAP consultants in the 90s making a very good living advising businesses how to set up and use these products? Salesforce.com is now that. And despite being "enterprise" they actually have very talented engineers. Also, critical mass. They have managed to build this ecosystem and everyone working in Corporate IT is drinking the kool aid. Google "Dreamforce 2016" if you want to see just how cult like it's all become. Like I say, they are going to replace Oracle. It's funny how much shared DNA they have though.