Renal failure is when the kidneys fail to function properly. It can broadly be divided into two categories - acute renal failure and chronic renal failure. 

Acute renal failure is, as the name implies, a rapidly progressive loss of renal function, generally characterised by oliguria (decreased urine production), body fluid disturbances and electrolyte derangement. An underlying cause must be identified to arrest the progress, and dialysis may be necessary to bridge the time gap required for treating these underlying causes. 

Chronic renal failure develops slowly and gives few symptoms initially. It can be the complication of a large number of kidney diseases, such as IgA nephritis, glomerulonephritis, chronic pyelonephritis and urinary retention. End-stage renal failure (ESRF) is the ultimate consequence, and dialysis is generally required while a donor for renal transplant is found.

see also renal hypertension