The Inquisitors are the secret police and hidden enforcers of the Emporers will. Swearing loyalty to the Imperium Of Man.They are  entrusted with the knowledge of the Inquisition (and all that that entails), Inquisitors are the hand and knife which slide from out of the shadows to strike at the Emperor's enemies.

The Inquisition is not a single group but is instead split into three core groups (Hereticus, Xenos, Malleus). Each person accepted into the Inquisition, and subsequently given Inquisitor training, is placed within these groups based upon their abilities and way of thinking.

The group to which the new Inquisitor is assigned is based upon which Ordos found them and whether their way of thinking is Puritan or Radical. No matter which group the Inquisitor is placed into, he must adhere to the strict doctrine and rules of the Inquisition:

Inquisition Doctrine
The word of the Emperor is law.
The best interests of the Imperium of Man are all that matters in life.
Only the Emperor knows what is truly in the best interests of the Imperium of Man.
Secrecy is a necessity which must be maintained at all costs.
The laws of man and of morality are nothing compared to the laws of the Inquisition.
Heretics must be found and judged.
Only Inquisitors are worthy of the knowledge and wisdom of what qualities make one a heretic.
The duty of an Inquisitor is but to serve the Emperor.

Written by me based on a piece of writing about Inquisitiors in an RPG which I found at http://www.interlog.com/~strust/wormwood/wwinquis.html

INQUISITOR'S APPRENTICE
The training of members of the Inquisition is a long and difficult process, with failure usually resulting in death, or worse. After the initial stages of testing and tuition the then initiate's training is given over to experienced Inquisitor's working the Emperor's will about the Imperium. Unfortunately the life of an Inquisitor is a hard one, and those that do not die often become corrupted by the daemonic influences they are constantly in contact with.
It is a commonly accepted fact that more apprentices die at the hands of their instructors than through any other means. Those that do survive their initiate training will be given into the care of an even more experienced instructor who will complete the penultimate part of their testing. It is not known what the apprentices must go through to become fully fledged Inquisitor's, but the screams which emanate from their training facilities on earth are surely testament to their difficulty.

http://www.wanderers-rest.co.uk/40k/imperial/is-hq-apprentice.htm

The Inquisition: The Benefits & How to Join
by Tang Chi Meng (tangcm@pl.jaring.my)

Once upon a time, in the far far future, on Imperium Tonight!, today's episode; "The Inquisition: The Benefits & How to Join."..

On holoscreen an old pudgy bespectacled man sits comfortably on a comfy chair.

" Hi, I want YOU " (points) " to join the Inquisition.. " he says while smiling a toothy grin.

" Now, I know what you're thinking. What benefits does one get from joining our rather secretive organization, infamously known as the left hand of the Emperor? There are many.." (grins) " Firstly, members of the Inquisition are bound by no Imperial law, one of the best perks of joining us. "

 Screen switches to the daily scenes of life of Inquisitor Schwarzenegger, where he cuts traffic on his special Codex: Imperial Agents jetbike, blows up people he doesn't like [and laughs while doing it too], jay-walking in Imperial Terminator armour without fear of getting run over by hover-transporters and 'relieving' himself against a wall of a Judge's Precinct.

" Secondly, having knowledge of mysterious and incomphrehensible beings, a definite hit, especially at those social gatherings. Impress people!.. "

Screen switches to Inquisitor Jaq Draco who speaks the names " Khorne, Tzeentch, Nurgle, Slaanesh " and his mates, a Callidus Assassin, Squat Guild Engineer, and Imperial Astropath sheepishly don't have a clue on what he's talking about.. [note: Smug look on Inquisitor Draco's face]

" Thirdly, instill terror and fear to all who stand before you, especially with your cool party tricks "

Screen switches to black armoured Inquisitor Vader, strangling an incompetent Imperial Navy Admiral [using his mind!woowww..] while others look on in terror. [note: Imperial March music playing in background]

" Fourthly, battle against dreaded and despicable enemies of the Emperor forever, not neccessarily a bad thing if you know what I mean.. "

Screen switches to scene where Inquisitor Bond " battles " against a handful of Daemonettes of Slaanesh.. [note: Secret Agent Man music playing in background..]

" Aha I see I have captivated your interest. Take a look at the following scenes to quench your unsatiable appetite for information.. " he says.

Screen fades and one by one, the following still images appear; the infamous weirdly-hair-styled Inquisitor Ventura stalking Wyrd Beastmasters in the dank underhive of planet Necromunda, the popular Cell of Inquisitor Mulder and Inquisitor Scully also known as the ' X - Holodata ' duo endlessly uncovering heinous conspiracies (Usually related to witchcraft, aliens, mutations or heresy) by planetary governments almost every lunar cycle, Inquisitor Holmes with his clueless partner Adeptus-Medic Watson investigating the case of 'The Flesh Hounds of Khorne of Baskerville Prime', and various other stills that form a rather nice slideshow. Old pudgy bespectacled man reappears.

" Requirements for joining are simple. If you have a knack for praying to the Emperor every now and then, investigating and reporting all manner of mutations, witchcraft, crimes and heresies conducted by your neighbours, plus generally making life misreable for others, you're welcome to join. Don't worry about contacting us, we'll contact you. Psionic ability will be a great plus too. "

Suddenly man frowns..

" Of course, if you're a religious zealot " (picture of Electro Tech-priest displayed.) " or/and a bloodthirsty maniac, " (suddenly picture fades, picture of Khorne Bezerker appears in place.) " we really don't need of your types. You can alternatively join the Adeptus Ministorum.. " (man reappears.) he says while clearing his throat.

" Well, what are you waiting for? Display your abilities and if you're lucky, you could join our prestigious and much-respected organization! Remember, I'm not only an Inquisitor, I'm also the Inquisitorial Representative and a High Lord of Terra, one of the most powerful men in the Imperium! " he says, waving, while screen fades....