Since "-itis" means inflammation, sinusitis refers to inflammation of the lining of the sinuses. 

Medically speaking, sinus infections are classified according to the sinuses involved.
For example, maxillary sinusitis indicates an infection of the maxillary sinus, located in the mid-face. 
Also must indicate which side is involved: right maxillary sinusitis, left maxillary sinusitis, or bilateral maxillary sinusitis when both sides are infected. 
Any number of your sinuses can be inflamed at one time. When all are infected, the term pansinusitis is applied.

Sinusitis is also classified by duration and frequency. What we refer to as "acute" sinusitis versus "chronic" sinusitis. Acute sinusitis lasts less than eight weeks, or occurs less than four times a year. Chronic sinusitis is a persistent disease of more than eight weeks duration, or more than four infections per year. Acute and chronic sinusitis are  different diseases with different symptoms and different courses of treatment.