Ultrafiltration is a form of filtration that uses membranes to preferentially separate different fluids or ions. Ultrafiltration also uses a membrane that is partially permeable to perform the separation, but the membrane's pores are typically much larger than the membrane pores that are used in reverse osmosis. 

Ultrafiltration is most commonly used to separate a solution that has a mixture of some desirable components and some that are not desirable.