shameless c & p from http://slf4j.org/news.html (February 25th, 2007 - Release of SLF4J 1.3.0)

This release consists of rearrangement of classes among projects. More specifically, the org.slf4j.LoggerFactory class is now packaged within the slf4j-api.jar file instead of the various slf4j bindings. It follows that client code needs to depend on only slf4j-api in order to compile, while the various slf4j bindings are only needed as runtime dependencies. See also the Maven2-related FAQ entry. Given the practical significance of this change, we highly recommend that library-authors upgrade to version 1.3 at their earliest convenience.