http://jsf.javabeat.net/articles/2007/06/jsf-unified-expression-language/

1) Introduction

Those of you who have been developing with JavaServer Pages (JSP) technology have probably used its simple expression language to access data from your JSP pages. Since the introduction of this expression language, other Java-based web technologies have been released and have become quite popular with the Java web application developer community.

One of these is the UI component framework, JavaServer Faces technology. To support its powerful features, this framework needed its own expression language (EL). However, this EL presented some problems when used with some JSP tags. Therefore, the specification writers and expert groups of the Java webtier technologies collaborated on a new, unified expression language to help align these technologies by adopting the features offered by the JavaServer Faces expression language. These features include the ability to use expressions to set the value of external object properties as well as get data from them and to invoke methods. At the same time, the webtier team made the language pluggable and extensible so that advanced developers can add custom resolvers able to evaluate expressions not already supported by the expression language.
2) Expression Language Beginnings

The unified expression language started out as the JavaServer Pages Standard Tag Library (JSTL), version 1.0 expression language, originally called SPEL (Simplest Possible Expression Language). True to its name, this expression language is easy-to-use way to access external data objects. For example, the expression ${customer.name} is used to look up the customer JavaBeans component and call the getName method to retrieve the value of the name property. This value is then rendered into the page.

By using this expression language, we can drastically reduce the amount of scripting in our pages, resulting in greater productivity, easier maintenance, and a flatter learning curve in terms of page development.