Here are the three major processes that form urine:
Filtration is when body fluids leak into the tubule. Everything except proteins, lipids, and cells can get in, so the urine at this point is very much like plasma. It is called ultrafiltrate.
Reabsorbtion is when cells of the renal tubule move compounds back into the blood. The cells reabsorb things like glucose, amino acids, and Na+. Water is reabsorbed too, because it follows these solutes by osmosis.
Secretion is when cells of the renal tubule move things from the blood into the urine. A good example of secretion is when the Na+/K+ ATPase turns on and moves K+ into the urine.
The kidney tubule that does all this is called a NEPHRON.