I am thinking about getting some new alloy wheels for my car. How do I find out what rim sizes or specifications are compatible with my car? Or any car for that matter?
The easiest way is to get wheels you know fit - i.e. ones specifically sold to fit a Passat ;)
Otherwise you'll need something compatible - the key things are the PCD (bolt pattern), offset (ET) and centre bore (CB above).
The bolt pattern is obvious, it's how many bolts you have (5) and how far apart they are (equally spaced around a 112mm diameter circle). Centre bore is the size of the hole in the centre of the wheel - if it's too small, the wheel won't go on, and if it's too big, it'll be a loose fit and put extra stress on the wheel bolts.
Offset is how far in or out the wheel sits - the more positive it is, the further into the wheelarch the wheel sits. The other numbers under 'Rim' above give the size of the wheel (e.g. 6jx15 is a 6 inch wide by 15 inch diameter wheel) - the combination of size and offset is important to ensure the wheel doesn't rub the inside of the wheelarch, or the suspension components.
The catch is that even if you get something with the right PCD, bore, size, and offset, it still might not fit - particularly on performance cars with big brakes... It's best to try a wheel on the car before buying...