The idea is simple, Suck all the fumes out the back with a fan, down a tube and out the window!
The one I went for is a Chinese model. Its cheap and cheerful but it works well. the unit itself comes with a tube that screws onto the back. if you are just wanting to extract particles then it will work perfectly well without the tube attached. But for fume extraction aswell, hang the tube out the window and it works very well.
This unit is not built like a tank. frankly its a little flimsy and care needs to be exercised if its not permanently mounted to the workbench. I simply don't have space for a dedicated painting area so mine gets packed away at the end of each session.
The extractor works OK. not great.... a little more airflow would be welcome but for a cheap and cheerful model, it does its job. and most importantly... it keeps the wife happy!
As with many Asian tools, the instructions have been written by a slightly dodgy translator! (probably with the help of Google Translate!)
If you are going to be doing lots of painting with solvent paints then I suggest you go for a better one. But for what I do, its ideal.