I am still trying to work it all out but it's a control box for the battery that's inside. You can adjust watts, temp, ohms and some other stuff.
Here are the basics I learned...
Some have removable batteries and some have internal fixed batteries.
The best names I have read about are Joyetech (sometimes spelled joytech), Eleaf and Reuleaux. So far I bought an Eleaf and a Joyetech eVic. I am looking at the Reuleaux for my third.
I didn't like the removable battery option. You might read a lot about these batteries blowing up and how dangerous they are. Because they pack so much power in a small package, yes, they can be dangerous. However, almost every story I read on the internet about blowing up were in mod boxes that were DIY or some other item that did not have the proper overcharge and release control protection. Any box mod that has a brand name should be fine.
Also, from what I have read most of these batteries are very low quality except the Sony and the (Either LG or Samsung, I forget. I think LG) Anyway, stay away from off brand batteries.
The first one I bought was the tiny Joyetech eVic basic. Really cheap. Like 20 something bux. It works great and has all the features of the bigger, more expensive models just...... a smaller battery so it doesn't last long. In my hand with my heavy use, only a few hours.
The second one I bought was an Eleaf iStick Power. It is a little bigger but has a 5000mAh battery and this puppy never seems to run down.
They have 2 and 3 and 4 battery models that I am looking at but they are about 80 buxs...... geeze, 80 bux for a battery??? LOL
Let me know if you have any specific questions.
If you are searching eBay or Amazon, search tags might be.... "box mod", "510 battery", "Eleaf", "Joyetech" . I am also looking at the iJoy but most of them have iJoy batteries and I heard those are crumby batteries.
I didn't want to hassle with the charger so I prefer the ones with fixed batteries. However, if I only had 1 box and only 1 box, I might go with the removable type just so I would never run out. With 2 or more boxes, I always have a spare on the charger.
User's choice, I guess.
Let me know if I can help.