[bruce511]

Public Domain is not Free Software (in the FSF sense) because it has none of the encumbrances of a Free Software license.

In other words you don't use PD software "like Free Software". You can use it in many places where Free Software would not be permissible.

In terms of -developer- freedom, public domain is top of the pile, the Open Source, then Free Software.

In terms of -user- freedoms Free Software is top of the pile, OSS in the middle, public domain is similar to commercial software.

[CorrectHorseBat]

Open source and Free software have different philosophies, but in practice they are essentially the same. You are thinking about copyleft vs non-copyleft. BSD, MIT, CC0 are all Free Software licenses but not copyleft.

[umanwizard]

You’re making the common mistake of confusing the copyleft vs. permissive distinction with the free software vs. open source distinction.

GPL is copyleft. MIT, BSD etc. are permissive. But all of those are both free software and open source, which are essentially synonyms.

The reason so many people get confused by this is that some of the people who prefer copyleft licenses (notably the FSF) also tend to prefer the term “free software”, for philosophical reasons.