What really is a headache?

All i keep reading is a headache is the most common form of pain, etc., etc. Can be caused from stress, muscle spasms, etc. Will someone just tell me what a normal headache, that you just get for the hell of it, is. Include medical jargon if you want, say something about vascular contraction something or other.

Headaches are usually caused by inflammation of the blood vessels in your brain. The inflammation draws blood to your brain and vascular contraction forces blood out of the blood vessels. Your head can't expand because your skull is hard. This puts pressure on your brain and this causes pain.
This is why people use antiinflammatory agents for headaches (Aspirin, Ibuprofen, Aleve). Serotonin is involved in the inflammatory cascade and antimigraine drugs like Imitrex work by modulating this pathway.

pain of unknown sources in your head

a pain you don't want, i have bad headaches

vascular constriction of the vessels in your head or upper neck

A pain is not normal, except the labor pains.
There are many types of headaches. It means, plainly : "pain in the head"
Vascular contraction seems to be the cause of migraine, a specific type of headache. There are different types of migraines, even including the body.
Stress headache is usually felt in the back of the head, obviously related to stress.