15" is the diameter, the J value is the width (stock is 6.5j for 15" alloys), 4 is the amount of boultholes, 100 is the distance between those holes. And the ET is the offset or how deep the mounting holes are in the alloy, not shure about that. But the stock et is somewhere around 40.
Plz correct me if I'm wrong guys, I know a little about it all but I'm not an expert...
If you intend to buy some, just ask if you can do a testfit of one alloy. And be sure you buy real BBS's and not some cheap chinacrap