A short overview of what the log4j source tree contains
org.apache.log4j : Is the main log4j classes and interfaces, in particular Appender (and implementations thereof), Layout, Level, Logger.
org.apache.log4j.spi : Contains part of the System Programming Interface (SPI) needed to extend log4j.
org.apache.log4j.chainsaw : A Swing-based GUI log viewer, see the chainsaw description.
org.apache.log4j.config : Package used in getting/setting component properties.
org.apache.log4j.helpers : Various classes only used internally by log4j.
org.apache.log4j.jdbc : An Appender to log events through a JDBC connection (generally, thus, to a database).
org.apache.log4j.jmx : Classes to let you configure log4j through JMX. It is unfortunately not of production quality.
org.apache.log4j.lf5 : Log Force 5 implementation contributed by ThoughtWorks. A Swing-based GUI which inspired chainsaw.
org.apache.log4j.net : Appenders for remote logging to JMS, to SMTP, over Socket to a remote log4j server, to Syslog or to Telnet.
org.apache.log4j.nt : Appender for logging to the Windows NT Event Log through Java Native Interface (with the associated C code).
org.apache.log4j.or : Helper classes for rendering Objects depending on their class type.
org.apache.log4j.performance : Performance test code.
org.apache.log4j.xml : Contains various XML-based components, in particular a class to initialize the log4j environment using a DOM tree.
org.apache.log4j.varia : Contains various appenders, filters and other odds and ends.