As others have noted, they are molecules that make up proteins. So not one thing at all, just a description of the type of molecule.

Most are being rushed to market in front of the science, and some are unavailable through legit channels so there's no guarantee what you are shooting up.

But the ones that are studied and work are really cool. GLP drugs have helped so many people, and the research keeps getting better and better on them. CBL-514 looks like a miracle - something that can dissolve abdominal fat preferentially, sparing all other tissues. I would love that - if I lose weight it leaves first from my legs & ass and boobs, stomach does not budge until I am a ruler shape. Sermorelin is pretty well studied too - growth hormone releasing hormone. Copper peptide speeds wound healing.

So I think they will become a drug category and get regulated, especially as people discover new ones and patent them. Right now it's sort of the wild west, people are likely harming themselves.