Most urinary tract infections (UTIs) involve only the bladder and urethra (the lower urinary system). Pyelonephritis results when a UTI progresses to involve the upper urinary system (the kidneys and ureters).
The kidneys filter the blood to produce urine. Two tubes called the ureters carry urine from the kidneys down to the bladder. Urine travels from the bladder out of the body through the urethra.
Urethritis is inflammation of the urethra. That's the tube that carries urine from the bladder to outside the body.
Pain with urination is the main symptom of urethritis. Urethritis is commonly due to infection by bacteria. It can typically be cured with antibiotics.