What exactly is nephritis and what are the causes?

A little vague term. The term nephritis generally means inflammation of the kidneys filtering units. The usual term is glomerulonephritis gn. However, whatever term you use is not adequate to describe the underlying mechanism of disease. That is why the full terms are membranous glomerulonephritis, minimal glomerulonephritis, hereditary nephritis etc. If you have nephritis ask you doctor for the full name/description.
Nephritis. This is a general term for a condition that involves the kidneys. The kidney is made of tubules which can be affected by high blood pressure, diabetes, and many other diseases and condition. This can cause microscopic blood in the urine, decrease kidney function and needs to be monitored on a regular basis by a primary care MD and sometimes a nephrologist.