A stock is a a full picture, like a wallpaper scaled down for signature size. You get better detail and better pictures and alot easier then renders, also with stocks it means no downloaded brushes. Your backgrounds are most usually made of filters for stocks they are more advanced and just dont look good with stocks you could mess up the whole picture
Renders are just your cutout pictures everyone starts off with, they dont have a background obviously and it is harder to get detail but still very possible you can take stock images and place them behind your render and work from there
Work of mine I recently did: The background was a dock and I used a render
You can fully see the background pretty well but dont know exactly what it is and yeah...I made my own stock basically. Anything else?
If Dinosaurs were still alive... wow i'd fly my pterodactyl eff cars