Nephrotic syndrome is characterized by proteinuria, hypoalbuminemia, edema, and hyperlipidemia. It can be primary, caused by diseases of the kidney itself, or secondary, caused by systemic illnesses that affect the kidneys. The most common primary causes are minimal-change disease in children and membranous glomerulonephritis in adults. Secondary causes include diabetes, lupus, and infections. Treatment involves controlling edema with diuretics, treating underlying conditions, and using steroids, immunosuppressants, or ACE inhibitors depending on disease type and severity.