To understand the conditions that affect your kidneys, it is helpful to understand the work they do. 
How your kidneys work
Your kidneys are sophisticated reprocessing machines. Every day, they process about 200 quarts of blood and sift out about 2 quarts of waste products and extra water. The wastes and extra water become urine. The removal of wastes occurs in tiny units inside the kidneys called nephrons. Each kidney has about a million nephrons. A complicated chemical exchange takes place, as waste materials and water leave the blood and enter the urinary system.

In addition to processing waste from your blood, your kidneys measure out the chemicals your body needs like sodium, phosphorus, and potassium. They are released back to the blood to be returned to the body. In this way your kidneys regulate the right balance of chemicals that are necessary for life.

Renal versus Kidney
"Renal" is a term referring to the kidney. Your doctor may use the word "renal" when discussing your condition. 

Kidney Stones
A kidney stone is a hard mass developed from crystals that separate from the urine. A kidney stone forms in the kidneys but can end up anywhere in your urinary tract. Normally, urine contains chemicals that prevent or inhibit the crystals from forming. When these inhibitors do not work, you may develop stones.
Kidney Infection or Pyelonephritis
Pyelonephritis is a kidney infection, usually from bacteria that have spread from the bladder.
Kidney Cysts
Kidney cysts are fluid-filled sacs that form in the kidneys. They're almost always noncancerous. Up to half of people older than age 50 have at least one kidney cyst.
Berger's Disease
The cause of IgA nephropathy is abnormal deposits of the immunoglobulin A (IgA) inside the tiny blood vessels (glomeruli) of your kidneys.
Polycystic Kidney Disease
Polycystic kidney disease (PKD) is a genetic disorder characterized by the growth of numerous cysts in the kidneys.
Acute Kidney Failure
Acute kidney failure is the sudden loss of your kidneys' ability to perform their main function.