The medical condition of suddenly increased pressure to the whole head due to massive quantities of mucus filling up the sinal passages. This condition has caused the most gruesome demise of many a valiant Kleenex or handkerchief who will allow themselves to be coated or filled with this infected snot for the relief of the afflicted person.

Characterized by wild amounts of pain in the face or like a headache, postnasal drip, and really disgusting colored snot (think the yellow and green on your can of Mountain Dew).

This condition may happen to one or more of the facial sinuses. It can be caused by abnormalities in the sufferer's physiology (small sinus clearing passageways), the common cold, and-or blowing your nose too hard.

Try a couple Sudafed, a couple Advil, and a humid environment (like sticking your head over a bowl of really hot water). Don't use affrin-type stuff; the rebound effect will murder your sinuses.

Oh, and this condition really, really hurts. Infection of the upper sinuses leads to monster headaches, infection of the lower sinuses causes toothaches and facial pain. So be careful when blowing your nose.

In addition to not blowing your nose too hard... don't hold in a sneeze, and don't fully occlude one nostril when blowing your nose (sometimes when you have a cold, this is so effective that you can't not do it... just don't do it too often).
Sinus infections are dangerous in that the infection has the possibility of somehow getting into your brain if it penetrates the blood-brain barrier. If the symptoms (which include chronic nasal congestion as well, if you didn't catch that) don't go away after about two weeks, go see a doctor.