What is the difference between Simple Logging Facade for Java and Apache Commons Logging?
From the SLF4J FAQ:
SLF4J is conceptually very similar to JCL. As such, it can be thought of as yet another logging facade. However, SLF4J is much simpler in design and arguably more robust. In a nutshell, SLF4J avoid the class loader issues that plague JCL.
Do a google for "JCL classloader issues" for more on this...