The function of the kidneys are to maintain the body's water content, sodium and potassium levels, acid-base level, and the level of certain body waste products.  Substances pass through the kidneys and leave the body in urine.  Kidney function is important both in what's removed from the body, and in what's saved.

Kidney disease refers to an abnormality or a degenerative process within the kidneys, which may or may not progress to kidney failure.  Because the kidneys have alot of reserve capacity, kidney disease does not result in kidney failure until more than 2/3 of the kidney tissue is non-functional.

Kidney failure refers to kidneys that cannot maintain the body's water content, acid-base level, or the level of body waste products.  It is not a specific disease entity, but refers to the impaired-functional state of the kidneys.  Kidney failure may be quick-onset, referred to as "acute", or develop over a long period of time, referred to as "chronic.  Kidney failure may be reversible or irreversible.

Although the kidneys have tremendous reserve capacity that it can call upon to take over for damaged areas, the damaged areas have only a limited ability to repair itself.  Damage to the kidneys that occurs over a long-period of time, causing gradual degeneration of more and more kidney tissue (chronic kidney failure) tends to be irreversible.  Sudden-onset (acute) kidney failure presents as much more severe disease, but if they survive the initial period, the disease is more likely to be reversible.