In the relatively short history of e-cigs only a few models have been introduced. Most models are produced in China at different levels of quality.

Pen Style (aka 901, sometimes called the DSE-901 or DSE901)

Size is similar to a pen and is the original size offered, often referred to as the 801 or 901 model.

Mini Style (aka DSE-801, 510, JOYE 510, NJOY PRO, Janty Dura-C, TECC Titan, Yeti 510, KR808D-1, VaporKing, Green Cig, Cigtechs)

Smaller than a pen but larger than a traditional cigarette the 510 models are commonly offered as the Janty Dura-C, Joye 510, JOYETECH, NJOY PRO, Titan and Yeti. These units currently are known to provide better vapor (better known to a newbie as smoke) and throat hit (the feeling of smoke within a human's throat and lungs) a pen style but in a smaller form. The KR808D-1, Vaporking, Inhalerette and several other mass marketed e-cigarettes are two piece "Mini Style" the Atomizer and cartridge are one piece. These models usually do not produce the same amount of vapor as a 510 series.

As a side note, the 510 models are noted to have the highest amount of knockoff (fake) imitation ecigs. These units can be dangerous, cause personal harm and even fires. Always buy from a trusted source. BE CAREFUL OF CHINA BASED SUPPLIERS as almost no regulations are in place.

M Series (aka 401, 403 400, Blu Cig, BluCigs)

Often referred to as the 403. The same size as an actual cigarette. Initially this model seems the most appealing unfortunately many reviews of the unit currently point out they cannot deliver the same amount of vapors (better known to a newbie as smoke) and throat hit (the feeling of smoke within a human's throat and lungs) as a real cigarette. Many opt for this unit and move back to smoking traditional cigarettes due to the lack of realism compared to tobacco cigarettes.

PCC
This is a very beneficial accessory PCC stands for Personal Charging Case. You charge the case via any USB connection. "On the Go" you can charge your ecig batteries several times before recharging via USB and / or AC outlet. Currently many e-cig units drain a battery in very short timeframe, the heavier the user the more recharging needed.

USB Pass-Through
Many units offer the USB pass-through (PTC) which is a great option for smokers who are often in front of a computer. One of the downsides of a good E-cig is battery life, The upside is a USB pass through unit receives direct power while using the unit. The down side is your "Cigarette" has a cord.

This is a good option for smokers who have successfully transitioned to an electronic cigarette and spend hours in front of a computer.