It's inflammation of the sinuses. It can be acute, which is typically inflammation caused by an infection (either viral or bacterial). Or, it can be chronic, which is inflammation caused by other factors (allergies, environmental irritants, etc.).
Typically the inflammation will cause your sinuses to get clogged by thick mucus that doesn't drain correctly which can lead to more infections, sinus pressure, a fever and congestion.
I think I might have that. For some reason I am only able to breathe through one nostril at a time and they switch every now and then. I have dealt with this pretty much my whole life as well. Thank god for nasal spray.