EL and UEL in JSP and JSF

JavaServer Pages (JSP) technology uses an expression language (EL) to access data from JSP pages. EL started out as the JavaServer Pages Standard Tag Library (JSTL), version 1.0 expression language, originally called SPEL (Simplest Possible Expression Language).

JavaServer Faces (JSF) technology enhances JSP with flexible and extensible UI component model, including:

• a set of classes that define UI components.
• a mechanism for handling events generated by the components.
• a process for converting and validating component data. 

A separate EL was created for JavaServer Faces Technology, capable of:

• wiring events generated by the components to server-side code.
• binding the components to server-side data objects.
• registering data validators and converters with the components.

Expression language used by the JSTL tags conflicted with that used by the component tags! Unifying these expression languages was highly desirable.

The new unified EL (UEL) essentially represents a union of the JSP and JavaServer Faces expression languages.

http://java.sun.com/products/jsp/reference/techart/unifiedEL.html