What is the main difference between an HO8 and a HO3?

An HO8 is a named peril policy, meaning you are only insured for the perils listed in the policy. An HO3 is an "all risk" policy, which means you are covered for all losses unless the peril is specifically excluded under the policy. As you can see the HO8 is a lot more restrictive. The premium may be cheaper but it offers less coverage.