Some have pouring lips; some have bases or embedded rubber rings on their underside specially designed to grip the counters (stainless ones rarely have this); some come with plastic cover sets for refrigerator storage.
Most people agree that you can’t have too many mixing bowls.
They can be made of plastic, glass, crockery / ceramic, or stainless.
Crockery / ceramic / pottery / stoneware Mixing Bowls can be the most beautiful and satisfying to work with. They are very heavy, and so very stable on the counter. However, not all of them are dishwasher proof, and they have to be thoroughly dry before you put them away or mould can grow and stain the glazing.
Glass ones are normally Pyrex or CorningWare. CorningWare ones are somewhat lighter than the Pyrex ones. They will both be heat proof except for direct heat. They are somewhat heavy, so mostly stable on the counter while in use. Some are actually very, very large measuring cups doubling as mixing bowls that also come with plastic covers for storage in the fridge. Glass Mixing Bowls tend to be the ones used most often in TV cooking shows — so that viewers can see inside the bowls.
Plastic / melamine Mixing Bowls can be harder to clean. Newer ones stand up to really hot ingredients better than the older ones did. It can be hard to get them to stop feeling greasy if you wash them by hand, and if they do feel greasy, it can be hard to get good beaten egg whites in them. They almost always suffer some kind of slight heat related accident relatively early in their lives, that will leave a slightly melted spot you’re staring at forever. They are, however, good for people, such as seniors, who might be losing strength in their arms so that they can’t manage heaving heavy bowls around any more.
Stainless Mixing Bowls are inexpensive, unbreakable, heat-proof and often come in sets of 3. They are very light, though, and can travel on the counter while you mixing in them.
Many people still swear by copper Mixing Bowls for beating eggs in. They are not dishwasher safe. Make sure that you don’t get the ones that are coated with lacquer for decorative display purposes only.