It refers to almost the same software. The difference is more on emphasis and what you care about than the set of software it refers to. It's about the underlying philosophy.

I say this because it's important to remember that open source is also intended to refer to a strict definition that also addresses user freedoms. The open source definition for example forbids discriminating against fields of endeavour and says the software should be usable for any purpose. "Open source" doesn't just mean visible source.

OpenBSD calls itself free software despite being opposed to copyleft, for example. Open source and free software are more allies than enemies, and I think it's important to remember that both want user freedom. They just express it in different ways and to a different degree.