db4o is a small object database, which enables easy object persistence. It addresses the problem of object-relational-mapping (impedance mismatch), and has a lot of handy features. A big advantage of db4o is, that there are no changes needed of exisiting classes. I think db4o is an easy way of object persistence which could be useful for prototyping or embedded applications. Since it's not JDO compliant, I don't think it's wise to use it for large applications.
rating comment: The article shows the main features of db4o, highlighted with a lot of snippets of code. What it doesn't address is the technical background of db4o, which I think would be interesting and useful to know.