This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
Databases have a limited number of connections available. When you close a JDBC connection, you tell the driver to release the database connection. If you don't, you may run out of database connections. What happens after that is implementation dependent, but generally not good.