Assuming you are talking about a standard residential shower, there are typically two types of showers - those with a curb, and those without. A curbless shower is one where the floor of the shower is flush with the rest of the bathroom floor, making it easier to step in and out of. This type of shower is becoming increasingly popular as it can give a more spacious feel to a small bathroom. However, curbed showers are still more common, and usually have a lip or threshold that you must step over in order to enter the shower. This lip can be anywhere from 1-3 inches high, and is there to contain water inside the shower so that it doesn't spill out onto the rest of the bathroom floor.