Apache Commons is a collection of libraries with many useful and reusable Java Components. Apache Commons project is maintained by Apache Software Foundation. The project is developed and maintained by developers of Apache community.
Apache Commons libraries are open source and can be obtained, modified and distributed freely (with preservation of copyright notice and disclaimer).
Apache Commons libraries provide functionality which is either missing from the core Java libraries or difficult to implement. These libraries are properly tested which is why you should use them instead of writing your own code where ever possible.
Apache commons provide so many useful libraries, some of them are listed below.
DbUtils library various JDBC and database related functionality.
Email library is very useful for sending emails using Java.
FileUpload library provides upload functionality to any web application.
IO provides input output utilities.
Lang library provides extra functionality to java.lang package.
Apache Commons libraries can be downloaded from Apache Commons Site