Identification

• Cysts are small, round fluid-filled sac within your kidneys. However, the cysts can be irregularly shaped. The lining of the sacs are thin and filled usually filled with clear or yellowish fluid.
  
  

Types

• There are two types of cysts: simple and complex. Simple cysts are noncancerous and most common.
  
  

Difference

• Complex cysts have a low risk of becoming cancerous, according to the Kidney Cancer Institute (KCI). They are considered complex because of different characteristics like irregular shapes, have a blood supply or have a thick lining.
  
  

Common Cause

• The most common cause of renal cysts, according to Family Doctor, is age. Typically, kidney cysts occur if you're 50 years or older.
  
  

Other Causes

• Other causes are polycystic kidney disease (PKD) and multicystic kidney dysplasia, according to the Mayo Clinic. These medical conditions can lead to renal, or kidney, failure.