The Inquisition. The Arm of the Church. Purgers of heresy, the literal bane of existence of anyone who is too vocal about his/her disagreements with official Church police or dogma for his/her own good.
The Inquisitors are essential a special breed of Cleric that does not focus on the spread of the word, but instead on the enforcement of official church doctrine among those the Church claims her own. That is not the only duty of inquisitors, however. In addition to heretics, their targets also include genuine demon-cultists, undead, and demons, or anyone who is affiliated with them and worships them.
As such, it comes to no surprise that Inquisitors are geared for fighting against threats of unholy nature, which includes wearing heavy armour as well as extensive training with weapons.
Just like Clerics, they use Clerical Magic. Inquisitors are also special in the sense that even if their personality is Lawful Neutral Lawful Evil or Neutral Evil - rather than any type of Good - they still use Light Magic, and receive no penalty at using it, even with low charisma.
Inquisitors are a prestige class of Clerics.
They are special in the sense that they are allowed to have an Evil alignment, but still rely on Light Magic and serve the Light Side.
Being an Inquisitor comes with the bonuses of receiving all armour skills, an advanced skill of blunt weapons, as well as the unique to enchant weapons against undead and demons.
In the video game adaptations of the FRPG, Inquisitors are in many ways halfway between Knights and Clerics, in the sense that Knights try to compromise between Warriors and Clerics the same way Battlemages try to compromise between Warriors and Magicians - as such, Inquisitors are somewhat more magic-oriented than Knights, and just like Clerics, they are forbidden from bladed weapons, limiting their selection of weapons to staffs, clubs, hammers, maces and morning stars. Nevertheless, they can wear all types of armour, they are slower and have less base attack than Knights, but have a bit higher base defense.
To reduce redundancy, in the 2013 game, only Humans were allowed to be Knights, while only High Elves and Halflings were allowed to be Inquisitors, which made no sense, as there have been several historical examples of Half-Elven or even High Elven Knights, and Inquisitors come from all races that happen to inhabit the Kingdoms of Froturn and Etrand.