Hellsing is a manga (Japanese comic series) by Kouta Hirano. It currently runs in the magazine Young King Ours (which features other manga series like Excel Saga and Trigun). There is also a 13-episode anime TV series produced by Gonzo and Pioneer that aired from late 2001 through early 2002, and a new OVA series more faithful to the manga began in February 2006.
The anime TV series is available in Japan on five DVDs. Outside of Japan, Pioneer released the series as four separate Region 1 DVDs, and the first DVD included a special collector's box. The OVA series is currently being released in Japan first, and then in North America. Currently, the manga is being translated into English by Dark Horse Comics.
The series takes place in modern-day England where Sir Integral Wingates Hellsing, the leader of Hellsing Organization, heads a private, secret army dedicated to the destruction of the undead. Integra is a member of the Round Table, and only the Queen of England and the other Knights know of Hellsing's existence.
As part of her army, Integra has two vampires, Arucard, who is bound to serve her, having once served her father, and Seras Victoria, a former police officer whom Arucard made into a vampire. Arucard is over 400 years old, and he is a true vampire. Besides his many powers as a vampire, he also uses guns to fight the undead.
As a note, the official spelling of Arucard is "Arucard" in both the manga and the anime. (Dark Horse Comics uses "Alucard" in its English translation.) In Japanese, "r" and "l" are the same sound and are represented by the same character, so either "r" or "l" can be used in romanization. However, Kouta Hirano specifically uses "Arucard" as the romanization, probably as a play on "Alucard" (which is, obviously, the backwards spelling of "Dracula").