Nephrology is kidney medicine. This is the best short explanation. Kidney medicine was actually its original name until about 1960.
The word Nephrology was made up from Greek word nephros (kidney) and the suffix -logy (the study of).
Nephrology is a specialty of medicine focused on the kidneys, kidney function and kidney disease. Nephrologists are the doctors who have done extra study in that speciality.
Nephrology seeks to maintain kidney health and treats kidney disease using lifestyle changes especially diet, medication and kidney replacement therapy.
Kidney replacement therapy is required when the kidneys stop working (kidney failure). There are two main options: dialysis and kidney transplantation.
Nephrologists treat whole body conditions affecting the kidneys such as diabetes, hypertension and immunological disease like lupus.
They also deal with conditions localised to the kidneys like kidney infection, kidney stones and drug toxicity to the kidneys.
Nephrology also deals with fluid, electrolytes (like sodium, potassium, calcium and chloride) and acid-base imbalances in the body.
Nephrologists treat complications due to kidney disease that affect other parts of the body such as renal bone disease, breathlessness, anaemia, oedema and hypertension.
Hypertension leads to kidney disease. Kidney disease causes hypertension. Nephrologists are quite possibly the best doctors to treat hypertension as they treat it every day.