I'll try to help you but this is a broad question.
Model numbers (e.g. 510, 306, 901 & 801) denote the threading size to the
device. With all the adapters, you can pretty much connect any atty to any
device. Also, each model number is a different overall size to the overall atomizer. However, airflow is a factor in the overall combination contributing to vapor, throat hit and vapor production. Different adapters offer different airflow - even within the same size - see my post comparing 510 to 801 adapters here:
http://www.e-cigarette-forum.com/fo...dapters-w-pics-airflow-attys-little-help.html
Each model is made by various manufacturers and there are differences between the various manufacturers. In some cases, like the 510, the model numbers do not indicate manufacturers and other model numbers (such as the 801 series) do sometimes reflect differences which may not be apparent at first. For example, in the 801 series you may find 801, 302, BE112 and 4072. 801s are usually made by SLB/DSE, BE112 are made by Best Ecig, 302 made by Joye and I do not know who makes the 4072. I have come across a Boge 302 but it is extremely rare. Each of these models has the same 801 style threads but very different air-flow intake designs and they each produce different vapor, TH and flavor.
Each model number, such as mentioned above 510, 306, 901 and 801 all come in various ohms ratings. so, many of these you can get in Low Resistance (LR) versions usually between 1.5 ohms and 2 ohms. Standard 510 attys are usually 2.5 ohms. Standard 901s and 801 series are usually between 3 to 3.3 ohms (some at 3.5 ohms). Then there are often HV versions which can vary between 3.5 and 5.2 ohms.
The ohms ratings, without getting overly technical, are a factor in Ohms Law and we match up an ohms ratings with the Volts we are using to achieve a desired Watts level (heat output). Generically speaking, you can use the formula Volts squared and then divide by ohms (R) to get the Watts a given combo will produce. Simply put: VxV/R=W However, this is not conclusive because Amps are not considered. But, you can use this as a general guideline. To see the full combinations, you can use this online Ohms Law calculator (put in 2 of the 4 variables and click calculate to see the other 2 variables):
Ohm's and Joule's Law Calculator
Generally, many feel the Watts sweet spot for most juices is around 8 to 8.5 watts. So, if we are using a 5 Volt device and a 3 ohms atty: (5V x 5V)/3=8.33 watts. Any volts level above 3.7 volts is usually considered HV (High Voltage). Some people only have 3.7 volts devices and we can "simulate" the HV vaping experience by using Low Resistance (LR) attys. So if we use a 1.7 ohms atty on a 3.7 volts device: (3.7 x 3.7)/1.7=8.05 watts. Personally, I find HV vaping 8 watts different from vaping LR 8 watts but other people have differing opinions.
Everything about vaping is very subjective. Some people love certain juices and others hate those same juices. Some people love 510 attys and other people do not like 510 attys, Same goes for 801 style attys, some like the SLB 801s and others like the Joye 302s. other people do not like 801 style attys at all. Basically, like juices, you have to try everything to see what YOU like.
I hope this helps. I know it seems like info overload. If I can help you more, PM me.