Picture two doormen. A named-peril policy works from a guest list: only the causes of loss it lists get in. An open-peril policy works from a "no" list: every cause of direct loss gets in unless an exclusion keeps it out.
"All-risk" is the older name for open-peril coverage. Despite the name, it does not cover everything: exclusions still apply. That is why "open peril" or "special form" is now the preferred term.
Same loss, two results: if an unusual cause damages two homes, the open-peril policy covers it unless it is excluded, while the named-peril policy does not cover it unless it is listed. A missing exclusion does not help the named-peril insured, because that cause was never granted in the first place.
Causes-of-loss forms are graded basic, broad and special, from narrowest to widest:
Basic: the shortest list of named perils.
Broad: the basic list plus more named perils; still limited to its list.
Special: open peril, covering any direct loss that is not excluded.