This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Commons Logging is an abstraction layer for logging libraries. The most common logging implementations are java.util.logging and log4j, but there are others as well. If you ever want to switch your code from using one to the other, you'll find that you need to touch most source files. Commons Logging abstracts that away, and lets you configure which logging implementation to use with a properties file - no source changes required.
It's widely used, although apparently Tomcat doesn't use it.