Prebiotics: Foods containing substances (mostly fibers) that we cannot digest, but which feed our good microbes.
Probiotics: Live bacteria that confer a health benefit when ingested. Found in raw fermented foods like sauerkraut, kimchi, yogurt, kombucha, miso, certain cheeses, pickles, kefir and probiotic pills.
Synbiotics: a combination of synergistic prebiotics and probiotics; usually in a supplement form
Psychobiotics: Live microorganisms that confer health benefits to people with psychiatric illnesses such as anxiety and depression. (A subset of probiotics.) Major genera include Lactobacillus and Bifidobacteria.
Postbiotics: The metabolic byproducts of probiotic bacteria such as acetate, butyrate, and propionate; these are short chain fatty acids which are used as energy by the host (us!)
Note that even in a healthy gut, pathogenic organisms (bacteria, fungi, viruses) exist; they are kept in check by the “good” guys.