Cystitis means inflammation of the bladder. The most common cause is a bacterial urinary tract infection (a "bladder infection"), though it can occasionally have non-infectious causes (such as interstitial cystitis).
Typical symptoms are burning when you urinate, a frequent and urgent need to go, passing only small amounts, cloudy urine, and lower-abdominal discomfort. Bacterial cystitis usually needs prescription antibiotics from a clinician — see urinary tract infection. Seek prompt care for fever, back pain, or blood in the urine.