Here's a breakdown:

* Beneficial bacteria: This is a general term for bacteria that provide a positive benefit to their host.

* Friendly bacteria: This term is often used interchangeably with beneficial bacteria.

* Probiotics: These are live microorganisms that, when administered in adequate amounts, provide a health benefit to the host. This is a specific category of beneficial bacteria often found in supplements and fermented foods.

* Commensal bacteria: These bacteria live in a symbiotic relationship with their host, meaning they benefit from the relationship without harming the host.

It's important to remember that "non-harmful" is not always the same as "beneficial." Some bacteria are simply neutral and have little to no impact on their host.