Hellsing is the story of the Hellsing organization lead by Integra (descendant of Van Helsing and current head of the organization), "Alucard" (Strongest vampire, Helsing's personal servant and protagonist of the serie) and Seras (former police woman now servant of Alucard and Integra). The story is set in great britain, suddently there is an augmentation in amount of vampires, furthermore they dont seem to be normal vampires. The Hellsing organization quickly comes into conflict with a mysterious group known as Millenium, which threatens to destroy them. The story eventually turns into a full fledged war between the Hellsing group and even the vatican and nazis, and Alucard takes the leading stage in the bloody battle.
The 1st anime done for Hellsing totally deviates from the original storyline and is thus non-canon, the Hellsing Ultimate OVA's are based of the original manga which is canon. Hellsing: The Dawn is also canon.
As a verse Hellsing is decent in power, but nowhere near as powerful as most people tend to overrate it. Pretty much every considerable fighter is hypersonic and has strength to rip through a human like paper, and even rip through metals as strong as steel. The biggest power house of the verse is Alucard, with a multitude of hax abilities and millions of lives within him (it is stated several times in the series that he relies on the lives hes absorbed to sustain his own and that true immortality does not exist). By the end of the series Alucard fuses with Schrödinger, which makes him nigh-omnipresent due to quantum mechanics, but this is a double edged sword due to Schrödingers powers working under quantum mechanics (reference: Schrödingers cat experiment) which makes Alucard capable of being defeated if he loses awareness of himself. Close to Alucard in power would only be Father Alexander Anderson (after absorbing Helena's nail), Walter C. Dorneaz (young) and "The Captain" (an original wolfman), these 3 and Alucard are pretty far above the rest of the verse in power. Overall the verse is low tier in this category.