The Struts Logic library is the switchyard that controls the flow of your JSP document. By testing for the presence and absence of data or its value (by condition or pattern matching) and evaluating your JSP based on it, you can produce a highly customized page that varies greatly depending on tests. You can also use the library to redirect control either to another page in this application or entirely outside the application.
You can also use the Logic library to iterate over collections. The subject of the next chapter, the Nested Tag library, can be of great utility when used with iterate.