Hellsing is a Japanise manga, that was later animated into the Hellsing TV series which are more popular. The action happens is in modern England and it is about the Hellsing organisation dedicated to the extermination of vampires and the protection of Great Britain.
The manga and the anime are different story-wise and character-wise. For example the main characters in the manga are Integral Hellsing and Alucard
, leaving Seras Victoria in the background, while the anime revolvs around Seras Victoria and her point of view about vampirism. Also Father Alexander Anderson has deeper personality in the manga than just a mindless killer as depicted in the anime.
Story-wise episodes 1, 2, 3 (the meeting with Father Anderson), 5&6 (the attack of the Hellsing HQ), and 10 (the flashback of how Alucard and Integral met) are close to the manga and the rest of the plot in the anime is totaly different.
Both the manga and the anime Hellsing are good. Buy either one of them (or both) and enter an amazing fantasy world!