What's the difference between store and shop?
No difference I can think of. "Store" is used more by Americans.
One more question about shops/ stores. On the Net, I met universal store as the equivalent for department store. Which is commonly used?
Department store is normal in America. I've never heard the term "universal store."
I believe it was originally a rural/urban difference. A shop was an urban boutique, whereas a store was a long-term rural storage facility mainly for dry goods. Both were where you bought your flower, sugar, coffee, etc. So store caught on in the Wild West and the rest of the US.
Excuse the non sequitur but I'm reminded of one of my favorite Far Side cartoons: