+6  A: 

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...

toolkit
+5  A: 

While SLF4J can be used as a facade over libraries like Log4j, or JUL, it can also be used as an API to implement native logging libraries, like Logback did. A native SLF4J library won't need adapter glue in the middle like the facade for Log4j or JUL does, so it should be more efficient as well.

jkf