Horror corruption is probably the nastiest non-fatal thing that can happen to a character in earthdawn; it principally affects the character in three types of way.
Effects of horror corruption
The first one is the Horror Mark a horrible way for the horror to affect the character this can be inflicted if the character uses raw magic (anywhere) or by a horror directly using their horror mark ability if they can see the character. This lasts a year and a day, but since a horror can inflict any non-damaging ability onto a character through a horror mark if they're within 100 miles of the horror they can pretty much keep doing this once a character is inflicted. The personal aspect of this as far as horrors are concerned is the abilities the horrors can use through this.
The second one is horror curses. These are a more personal aspect of horrors, as those horrors with the curse ability will have one that is personal to them. The most common one of these is cursed luck but there's a lot of scope for making these individual. These are the easiest way for horrors to inflict corruption on people as they can be placed on areas and items.
The third one is thought worm, although not a corruption par-se this allows the horror to force characters to do things or take damage for a year and a day, slowly corrupting them to do nasty deeds. Horrors need line of sight for these.
It's really worth tracking down the Horrors book as it goes into this in more detail and also gives you a lot more examples of the variety of horrors.
Horrors are earthdawns ultimate bogeymen, they are powerful, malevolent and extremely difficult to kill, even a small horror can provide a huge challenge for a moderately powerful party.
Notes from the horror book (I found my copy)
The Horror Mark described in the ED basic book is the "standard" horror mark, certain powerful horrors have their own versions of that power - examples are in this book. The book also expands on the horror powers a little more as well as the horror mark ability.