Public domain, other than by copyright expiration or (in the US) federal government creation is actually not clearly possible to create in a legally unambiguous way in many regimes with automatic copyright and no direct provision for unilateral transfer to the public domain; the most permissive open source licenses approximate it while providing better legal clarity.
Open source licenses often seek to protect the originator from liability they might otherwise incur from chain-of-commerce relationships, making it safer for them to make the software available without fee, and to prevent misattribution of origin. Other open source licenses, while not having field of use limits, seek to assure that derivatives are also offered on similarly-free terms. There's quite a variety, within the bounds of certain essential guarantees that are central to the common value proposition.
Sounds like public domain. What’s the difference between the two then?