The correct spelling is aneurism. It is a weakness in the wall of an artery, whic balloons out and may eventually burst, which can cause serious problems. They can occur anywhere in the body, so there aren't any overall symptoms. If one occurs in the brain, it can cause a very, very severe headache. If one occurs in the abdomen, it can cause pain there. However, they are very rare, so it's really not something that you should be worrying about. Sure, people die from them, but people also die from car accidents, and you still leave your house (I assume).
I don't think there's anything specific that causes them. Usually they are present for a very long time before they actually rupture. From the one case that I have heard of, the headache comes on very suddenly and is horribly debilitating, like hardly being able to dial 911 before passing out.
Of course, if you have never seen a doctor to be diagnosed with migraines, then you might bring it up at your next appointment, just to make sure that your headaches are indeed migraines.