A mixing bowl is a deep bowl that is particularly well suited for mixing ingredients together in.
These come in many materials, such as stainless steel, ceramic, glass, and plastic. If using a handheld electric mixer rather than a standing mixer, be sure to get mixing bowls that will be usable with such a mixer, and which have high enough sides to prevent splatter from it.
Typically one should have at least three mixing bowls of different sizes, as it is often necessary when baking to keep mixtures of ingredients separate. Also, prepared dough can be stored in the refrigerator directly in the mixing bowl, so consider bowls which can handle temperature change well and/or which have lids for such storage (if your bowls do not have lids, plastic wrap or aluminum foil may be used to cover them).