How do guns work?

Got this idea from another question about guns. Having lived my whole life in Europe, I really have no clue. I tried wikipedia and figured that some guns need gunpowder before you put the bullet and others don’t. Also that guns come in different sizes and therefore need different bullets.
(I was a nurse in the army so I never needed or wanted to touch an actual rifle).

No, don’t worry, I’m not buying one (yet).

And please remember I’m stupid, so don’t start using words like “calibre”. Just the basics.