Nephrology is the branch of medicine that deals with the health of the kidneys.

What does a nephrologist do?

A nephrologist is a doctor who specializes in diagnosing and treating diseases that affect the kidneys.
Following a diagnosis of renal disease a nephrologist may prescribe tests to assess the seriousness and identify the most appropriate treatment. Often the doctor will refer the patient to a dietitian or other professionals that can help the patient better manage the disease.

What diseases are treated by a nephrologist?

A nephrologist deals with glomerular and vascular disorders affecting the function of the kidneys, or the vessels associated with them, tubular and interstitial disorders affecting the kidney tubules and surrounding tissues, and diseases of the metabolism of minerals.
In particular the diseases include kidney stones, acute and chronic renal failure, hypertension, chronic kidney disease, and polycystic kidney disease. 

What procedures are used by a nephrologist?

A nephrologist will initially gather a patient's medical history including family diseases, and then conduct a physical examination. Further investigations may need to be prescribed such as blood analysis, urinalysis, biopsy, renal ultrasonography, and radiographs. 
The purely surgical procedures, however, are handled by a urologist.

When should a patient visit a nephrologist?

Typically a patient will be send to a nephrologist by a general practitioner for concerns of kidney problems, hypertension, and metabolic issues. In general, consulting this specialist is useful when the patient is dealing with kidney stones, chronic or repeated infections of the urinary tract, acute renal failure, uncontrolled hypertension, CKD in stages 4 or 5, presence of protein or blood in the urine, reduced renal function, and a glomerular filtration rate equal to or less than 30.