As of September 2015, Stack Exchange, Inc. rebranded itself as Stack Overflow as part of a broader business refocusing.
The "Stack Overflow" you see refers to the company "Stack Overflow" (Stack Exchange, Inc. d/b/a Stack Overflow).
Personally, I find this confusing, because the same name is used for both the company and the flagship site. For this reason, I generally refer to them using their legal name rather than their brand name.
Side note: part of why Stack Exchange, Inc. kept its existing legal name rather than change it was because there's another company with that name in New York state. For this reason, in its early years, it was named Stack Overflow Internet Services, Inc.