Diagnosing head gasket symptoms - Head gasket failure types

Someone I know was worried the head gasket in their car had blown.
That's usually a right reason to worry as this gasket plays a major role in the running of the engine and usually entails costly repairs.
I look for some informations as to how to understand why it happens and how it happens.
This link explains the various head gasket failure types and this one gives infos about blown head gasket symptoms.

Hope that helps!