Laws of Shabbat - 2

Different cooking vessels (the specific halachic ramifications will be discussed later on).

Any food in the primary vessel used for cooking, known as Kli Rishon, whether it is on the fire (stove,oven, coffee urn) or off it, is considered to be cooking the item. Others say that even if the temperature were below 45 degrees Celcius, it would be considered cooking (the stringency results from fear that people will leave the food on the fire until it cooks) [Mishnah Berurah].

The next level is pouring from the primary vessel (Erui Kli rishon), and it is the process of pouring the food or liquid onto another food or into another liquid.

Next, there is Kli Sheni, the secondary vessel. Something becomes a status of Kli Sheni when something from the primary vessel is poured into it, whether food or liquid.

Then we have the category of Erui Kli Sheni, pouring from the secondary vessel. This only exists for liquid, not for food.

Finally, there is a category of Kli shlishi, the third vessel, which is basically the vessel that one pours liquid from a kli sheni into.