The cauldron is a very versatile item! A witch should try to come about several various sizes! They can be used for anything from placing offerings in them to actually brewing "potions"! Like the challis, the cauldron represents the womb of the Goddess. It is also a symbol of earth and water.
You should have one small enough to fit on your altar. It can be used for incense and mixing small amounts of oils and liquids. It can be used to hold candles and also for scrying. A good medium sized cauldron is also good for the same thing just on a larger scale! I have a large cauldron I use for making what I call "Coven Stew"! That is a stew I make for gatherings, where I fill it with water and place it over a fire, I then ask every member who attends to bring an ingredient or two to add to the pot. By the time the ritual is finished and we are ready to feast we have a delicious pot luck! It can also be a place to burn petitions to the Goddess and God.
When selecting a cauldron select one made of cast iron because it can be placed over a fire. It should be free of rust (a small amount is expected with cast iron, but when washed and dried properly it should come off), smooth and have a slightly shiny surface. It should have a handle for carrying and hanging. It is good to find one with legs so it can sit off of surfaces but this is not a requirement. The best way to care for your cauldron is to wash and dry it by hand then place it over heat to completely dry it. Then rub it down with a good amount of cooking oil to prevent any rust. After first washing it you should never wash it with soap again! If it needs a good scrubbing use some water and a small amount of salt and then heat dry and oil it again. It would be nice to have a lid that fits it but it can be hard to find used cauldrons that still have lids and the new ones are expensive! I got my larger ones from my grandmother (the best chili was made in them over camp fires) and the lids are long gone!
The cauldron is associated with Cerridwen, who had a cauldron brew that took a year and a day to make. Odin is said to have received knowledge from a cauldron about battles.