ADMIN NOTE: This post is for those who wanted to know more about trigger warnings and their purpose. Thank you to outori for putting these resources together!
Here is an article on triggers, and why trigger warnings are good things.
Here is an article from someone who is against such warnings, and lots of people commenting on why it rubbed them the wrong way.
Also here is a nice one from someone who chooses to use them.
You could also read on trauma triggers on Wikipedia. The article is very short, but it suggests further reading.