distribued database

Your question implies that there is a definitive model of a computer cluster. That is not exactly the case. If you look at sourceforge.net under the heading of System-->Clusters you will see many projects. These projects cover a range of models that the project developers thought qualified as clustering. Additionally there are many projects under the parallel heading of System-->Grid Computing. Grid computing is, however, one of the models of a cluster.

In general I suppose you could say that a computer cluster is a group of computers that all run a shared system level process by which the computers communicate and share resources. A database server is an application level product. So, by a mere technicality, a distributed database server would not qualify as a grid computing cluster. In all other respects it acts just like a grid computing cluster.

And, that distinction is only valid if you accept the questionable definition that I provided of a cluster. These days the term 'cluster' is hard to pin down.