This section will be a brief intro to JDBC. It will talk about the basics of JDBC, issues, caveats, and so forth, but won't go into detail about the specifics of JDBC. The JDBC website has many good resources, and will always provide the most up to date information. Also, the API documentation included with your JDK has detailed information on specific classes, methods, and fields. Look for the "java.sql" package.
JDBC has classes to represent most of the basic pieces of a program's interaction with SQL--Connections, Statements, ResultSets, Blobs, and Clobs--all of which map directly to some concept in SQL. It also has has helper classes that represent meta-information, such as ResultSetMetaData, and DatabaseMetaData. These are useful for when you'd like to get the capabilities of the database, or the types of results returned, either for debugging, or because you don't know about the data you are dealing with. PostgreSQL's JDBC interface also provides classes to map to PostgreSQL's non-standard extensions to SQL and JDBC- Fastpath, Geometric types, Native Large Objects, and a class that aids serialization of Java Objects into the database.