What are the differences between positive and negative databases?

There are two different contexts is which these terms are used; the first is regarding "reputation", a positive database contains entries correponding to entities with good, or at least not bad, reputation. A negative database has the entries corresponding to entities with decidedly bad reputation.

In the second context, a positive database is what you normally thing a database to be, say entries containing names and credit-card numbers; a negative database contains all of the possible entries not included in the positive database, i.e., all possible name-creditcard combination except the ones in the positive DB. In fact a negative database has all possible character combinations (or for that matter bit combinations) of a given length that are not present in the negative db. This scheme has a potential for data security and personal privacy.