EPL is a pattern matching language built on top of EOL. EPL provides facilities for performing mattern matching on a model and produces a match model than can be used with the other Epsilon langauges in order to query, evaluate, tranform, etc. the found matches. A common scenario is to use the pattern matches as input for model transformations.
Chapter 11 of the Epsilon book provides a complete reference of the syntax and semantics of EPL.