It's pretty vague in the book, but generally it has to be something REALLY evil to give someone a CP. Encountering a Deamon, unless you are fighting it, or being in the presence of ancient, evil artefacts forces a Corruption Save.

It all starts on page 189 in the Atlatean Edition. Read up. :wink:
Most of the time you gain a corruption point by failing a corruption save. There are a few cases where you gain a point without getting to make a save (ie. the first time you cast Demonic Pact)

So the next question is; "what provokes a corruption save"?

By the RAW you provoke a corruption save by making peacful contact with a corrupting influence (ie. a demon or somebody with a Corruption Score of 10+) or by being power-drained in a War of the Souls by such a corrupting influence. There are also a few other specific sources called out (such as living in an area with Green Fire Stones).

However the rules also suggest, though they do not state, that other sources of corruption may exist including using corrupting magic items and participating in unspecified corrupting behaviours. I encourage GM's to personalize their own games by calling for more or fewer corruption saves depending on the tone they want for their game.

Also, you may want to read this thread for more info.

Hope that helps.