Here is a good explanation and some examples, as it relates to EJBs.
http://www.developer.com/java/other/article.php/734191
I have always thought of helper classes as classes which do the preliminary work, steps other than updating the database or handling transactions. This allows the
EJB (or whatever framework you are using) classes that do the heavy-duty work to be shorter, simpler, and easier to read/maintain/debug.