So, inferos and infernos are actually just a variation on the same word, with the "n" having been added later.  In the latter usage, it GENERALLY was thought to be synonymous with "Hell", but it's really the same word.  In English we have the derivative word "infernal", which generally refers to Hell proper and the fires of Hell, and it's why some people think that Christ descended into the actual Hell of the damned, but He did not.  I'm not sure how the "n" got added over time, but most official versions (e.g. from Trent) have it without the "n" ... as "inferos".