During Pesach, it is forbidden to use the same cooking utensils or tableware that was used year-round for Chametz. Even when the vessels are superficially clean, the Chametz absorbed in the walls, will be reabsorbed in the Pesach food (noten ta'am). It is customary and most recommendable, then, to have a separate set of cookware and tableware for Pesach. If this is not possible, one can still use some of the year round utensils after a process called:hag'ala, which is a form of sterilization that serves to eliminate any particles of Chametz absorbed in the utensils' walls.
All types of hag'ala follow one single principle: kebol'o kakh polto, which means that the expulsion of any substance absorbed in the walls of a utensil, will occur in the same way that those particles were absorbed there in the first place. The process of hag'ala, then, is different for each type of utensil, depending on the material from which it is made, the way it is normally used, way of cooking, etc.
Some examples of hag'ala
Glass: According to the Sephardic Minhag, any clear-glass utensils like cups or plates used throughout the year for Chametz should be thoroughly washed and they can then be used for Pesach without any further hag'ala. (Since glass does not absorb, it would not reabsorb back any Chametz particles).
Metal: Forks, spoons or knives should be thoroughly cleaned and then immersed in a pot of boiling water. Then, they are washed with cold water and can be used for Pesach. It is preferable, wherever possible, to perform the hag'ala only after the utensils have not been used for twenty-four hours.
Porcelain: Ceramic, pottery or porcelain china and tableware which was used throughout the year for Chametz are not suitable for Pesach. In other words hag'ala does not sterilize them; therefore they cannot be used for Pesach.
These are only some examples of hag'ala. Ask a Rabbi for more details.