Java Database Connectivity (JDBC) is a set of Java classes used to databases.
You can access any data source that you can obtain a JDBC driver for. What you can do with a particular database through JDBC depends on what the driver developer has implemented. See the Sun Microsystems JDBC site (http://java.sun.com/products/jdbc/) for more information.
Squeak can access a variety of databases as well. See the current listings on SqueakMap for more information. J2SBridge makes JDBC connectivity available for squeak programmers.