Corruption

Contrary to popular opinion, Aberrants (especially Grand ones) do not 'simply' bring madness to people. This is merely one aspect of effects caused by their eldritch natures on mortal beings, and one not even most important (although definitely most widespread).
This effects can be caused by reading forbidden books, conscious corruptive acts of already established Aberrant-worshipping groups (with their lairs – sometimes whole towns on the verges of civilization – slowly changing visitors), being in possesion of certain artifacts and objects, or being exposed to view of sufficiently eldritch beings.
Especially Pentagram is known for spreading the Corruption far and wide, but also other sufficiently strong while independent beings of Beyond can cause some degree of it.
During long and ardurous research, Imperial Magic Guild' specialists managed to divide the effects into four main types. Whose intensivity varies as some creatures 'prefer' certain types of corruption.

Depravity

We found the kidnapped boy. Some cultists used him as a sacrifice during ritual to summon lesser daemon of the Pentagram. The state of the body was... let's just say that I can't even imagine what he went through before they finally killed him.

Adventurer on one of his past jobs.

Probably the most terrifying type of effect that can touch mortal beings that came too close to beings of eldritch nature. Depravity can be best described as mental change that makes a person become mentally closer to the being in question. In most cases it means the mortal slowly changing into disgusting degenerate, commiting evil simply for evil... while feeling great pleasure from it.
Mortal that is sufficiently Depraved will willingly engage in most debauched and disgusting pleasures, including getting aroused merely from commiting such depraved acts – the more depraved and abominable, the better the 'reward'. The change starts subtle, with many not discovering they are under such effect until the changes aren't rather expansive. As a questionable advantage it makes one better attuned with eldritch powers, increasing power of doom magic and resistant to madness.
Depravity, as most effects, can be purged if one reacts in time. Most religious organisations have necessary specialists capable of exorcising Aberrant-induced corruption, with most 'talented' being either those of monotheist religions (for example visenian Lady, khardic Overtyrant or inrithism) and certain gods of polytheisms that are somehow considered a bane for enigmas (for example Shimmer and Shadow in Imperial Religion).
There is however a single 'but'. If the change was welcomed willingly – in other words if one willingly joined a cult of such beings or ceased resisting the changes, the chances to get rid of corruption fall down significantly. In fact such actions increase the lowest possible level of such corruption that can be achieved via exorcisms. There are still ways to make one's soul pure, but they involve – in most cases – things like burning on a stake (ceremoniously 'wiping out' the unworthy flesh that is the vessel of corruption), which makes it rather hard to achieve.

Madness

IT IS COMING, IT IS COMING!!!

Insane adventurer during mission in Valley of Ashes.

Madness is second type of corruption brought by contacts with Grand Aberrants and, sometimes, also lesser beings (which is mostly dependant on lenght of such contact and one's willpower). Madness of Beyond is something completely different but also in a way similar to 'typical' types of insanity.
Typical insanity happens when one's brain undergoes damage that somehow impairs its ability to reflect emotions and/or its connection to soul. The first type causes for example rage and agression (or lethargy), the second hallucinations or paranoia. This can be described best by comparison to a complicated machinery in which some of the cables and pipes are somehow faulty. Small holes, leaky seals, cut off cables and so on.
Corruption-induced madness is like (depending on the strenght of it) letting in a maniac with a wrench (that then unscrews anything he can) with a chainsaw (to ruin it completely) or a bomb. It is much deeper, making one's mind increasingly dysfunctional in a similar, but much faster and stronger way. While some effects (for example paranoia) are similar, it is simply because the maniac accidentally disrupted the same pipe or cable. The fundamental type of damage is different and much more severe, but works in a more or less similar way.
Thankfully mortal minds are a wonderful creation, possesing near inexhaustible self-regeneration abilities, in a way working better if the damages are stronger than 'mundane' disorders (because they are easier to noticed by this mechanism than small leaks and so on). Even after a maniac with a chainsaw was let in, it can still rebuild itself. Of course, it won't be the same as before – common longterm symptoms are anxiety, fear of the dark, partial amnesia, obssesive-compulsive disorders, alcoholism, nightmares and so on. But it will still exist.
Of course this also depends on certain factors. For example strongest beings will make suffiiciently weak minds crash completely, with no chances of getting back (it can be compared to detonating bomb in one's mind). Longterm exposure for relatively low levels of it – for example spending weeks in Pentagram-corrupted town – will also crash one's mind simply by changing it too slowly to trigger mind's defend & rebuild 'protocols'. Going back from the brink works best when for example groups of adventurers fight daemon of Pandaemonium – even if some of them changes into screaming madmen, they will recuperate if given enough time.

Taint

Strange. I could swear he had blue eyes, not silver.

Adventurer after meeting his collegue for the first time in years.

Taint is a rather elusive factor of Beyond corruption, that can be discerned only at the level of Grant Aberrants. That is also often mistaken with Mutation-type corruption, especially by those without professional knowledge.
Taint can be described as being slowly rewritten genetically (and anatomically) into certain 'preferred' state by the corruption spread by Grand Aberrants. It is generally differentiated from Mutation by not being debilitating in a direct way, while still leaving the corrupted being as one that can function in a society. Border between the two is often rather elusive and tends to be really hard to discern.
Generally speaking Taint is unique to every Grand Aberrant, causing completely different changes. It can be something rather minor – like change of eye color – or something exactly different, like complete gender change. The stronger the Aberrant, the stronger the Taint it causes, but the longer it takes for the changes to occure, as the body resists it the stronger the bigger they are.
Taint is also partially a mental thing, causing general dislike for religious places (or even pain while visiting), making one more attune to Depravity and doom magic, increasing damage received from magic (especially ne made to combat Pandaemonium) while slightly increasing resistance to Madness. It also sometimes work (especially on willing subjects) as a perequisite for Mutation.
The Taint can touch both willing participants and those unwilling. First is mainly the case of cultists that are 'rewarded' with it, the second by beings that are long-termly exposed to the corruption. Cultists can be changed very quickly, but it depends on the Grand Aberrant and how much is it ready to risk somebody noticing he is playing in the Light. Those exposed and generally having negative feelings towards it will be much harder to corrupt, unless they begin to somehow 'like' what is happening to them.
Purging the Taint is extremely hard. Low level of changes can be reverted simply by leaving sphere of influence of the Grand Aberrant (with exorcism making the reversing much faster). Higher levels of Taint are at least physically irreversible, with proper rituals being able to only purge the mental part of it.

Mutation

In front of him stood Numerius. Or what was left of him. When the abominable, inhuman... thing moved, he saw eyes of his old friend looking at him, silently begging for help.
Edvar screamed from the top of his lungs.

Excerpt from a horror novel published by a retired magician.

Mutation is the rarest and most feared type of corruption that in most cases happens to those that were deamed too strong to corrupt them mentally, rather than that adapting more physical approach. It requires direct, physical contact to initiate - be it a wound from specially cursed weapon, drinking some rather unsafe liquid, or things like rape.
From then one a victim is slowly changing, firstly rather marginal - color and texture of skin, colour of eyes and so on - finally leading to more 'complex' changes, in most cases changing a person into a sane but terrified mind bound to a gruesome and terrifying uncoherent blobs of flesh. A certain variant of this occurs when Aberrant is sufficiently strong and is capable of warping its cultists into rather inhuman creatures. They do it willingly, to become 'better' servants, better beings, closer to their overlord. This is also considered a mutation, simply guided one.
The change can be stopped via advanced alchemy or exorcisms, but reverting it is next to impossible. If the changes go far enough to make a person incapable of living, mercy kill is often employed. If the changes are small enough, they can be fixed, at least to a point.