Exploratory Testing

How do you define exploratory testing? Shall we involve test cases, it is kind of grey area for me.  Somewhere it is defined as "Exploring the software to discover its functionalities and drawbacks is what exploratory software testing does. So, this is a formal testing process that doesn't rely on test cases or test planning documents to test the application. Instead, testers go through the application and learn about its functionalities. They then, use exploratory test charters to direct, record and keep track of the exploratory test session’s observations."

I would appreciate if someone enlighten me on this.