E-cigarettes come in all shapes and sizes. If we should categorize E-cigarettes, we can first look at the way of inhalation. Many of the smaller E-cigarettes have a limited amount of air supply so that they are often inhaled mouth-to-lung (often referred to as MTL, or mouth to lung), just like traditional cigarettes. In contrast, there are devices that have a large amount of air supply. These devices are often inhaled 'directly' over the lungs (often referred to as DL, or direct lung). This latter category is in some ways comparable to smoking a water pipe. Often, we see large batteries in this latter category that can provide more power, firmer tanks that can handle this force and produce more vapor.
Of course, there are many different devices within these two categories, each with their own plus and minus points. There are also devices that float between categories (often labeled as RL, or restricted lung), and there are always exceptions. There are also smaller devices that are more near to DL in terms of inhalation and larger devices that are more near to a cigarette (MTL) in terms of inhalation.