Acute: recent. In gral. "acute" means of "recent" origen, and "chronic" means "old" or less recent. Some classifications use 6 months for "subacute". Chronic renal failure is insidious and depends upon the cause and how fast it gets worst. Acute renal failure usually is caused by low fluid volume in the body either dehydration or bleeding or it may be caused by toxins (poison, medications , dye contrast).
Length of time. Acute kidney failure happens rapidly likely in a period of hours to days. Most commonly, it occurs in hospitalized patient who have other medical problems such as suffering a heart attack or having an infectious problem. Chronic kidney failure occurs over months to years. It occurs in patients with long standing problems like diabetes and hypertension.