A User Defined Function Block or UDFB is a function, either written in the Ladder or C Editor, that is designed to perform a specific task, such as control an oven. Any function can be used as a UDFB, with the single exception of the Main function.
In addition to its code, a UDFB may include any of the following:

• Input and output tags to transfer values into and out of the function.
• Local tags, which only exist within that function.
• Global tags, structs, and arrays.

To use a UDFB just Drag & Drop it into the desired function, then link tags to the Function IN and Function Out parameters. Note that you can display the parameter description by mousing over it.

You can access a video tutorial via this link: 

http://www.youtube.com/watch?v=rb5WyCztLWw&list=PLFBq_OH6_be7snAZAycZzQMvAWjHkX9Pz