Active transport uses energy to transport a substance. Passive transport doesnt.
When the body's blood passes through the nephron's (a functional unit of the kidney) glomeralus (forgot how to spell it hehe), passive filtration occurs and the filtrate gets collected in the Bowman's Capsule. I think it also uses active transport to bring in unwanted substances that were not collected through that passive filtration. Anyhow, water in the filtrate is transported into or out of the kidney via passive osmosis (the degree of osmosis is controlled by adjusting the permeability of the kidney walls using ADH, ADH being released according to the body's needs). Salt and other substances in the filtrate is transported using active transport. Finally, The remains of the filtrate is secreted... which is active I think.
I think thats how it goes...