Logback is intended as a successor to the popular log4j project. At present
time, logback is divided into three modules, logback-core, logback-classic
and logback-access.

The logback-core module lays the groundwork for the other two modules. The
logback-classic module can be assimilated to a significantly improved
version of log4j. Moreover, logback-classic natively implements the SLF4J
API so that you can readily switch back and forth between logback and other
logging frameworks such as log4j or java.util.logging (JUL).

The logback-access module integrates with Servlet containers, such as
Tomcat and Jetty, to provide HTTP-access log functionality. Note that you
could easily build your own module on top of logback-core.