What is the difference between object-oriented database and relational database?

A Relational Database is designed and maintained following some very well defined rules of logic and algebra. It often portrays a "one to many" relationship between two sets of data, and, less often, a "one to one" and "many to many" relation can be developed. An OO database uses less rigid design parameters, and can be adjusted design-wise to fit almost any kind of data environment. In fact, I'm not absolutely sure there is such a thing as an "object oriented" database, so much as there are database objects that are created and maintained with OO programming. I know that sounds self-referenceing, but that's OO for you... A relational database uses structure to locate and display data values, rather than programming logic. With a correctly designed RDB, finding and displaying data is very simple, compared to earlier network databases. Relational databases also permit the use of JOINS to merge and match sets of data ("relations"), to glean more information from your database that would normally be available.