An engine to query streams of events using select statements over time windows and pattern expressions.
(by streams of events you need to think continuous queries vs traditional store and later query analytics)
Esper is actually much more capable - especially in the just released Esper 3.2, that for example brings support for a subset of nascent not finalized ANSI SQL standard match_recognize.
The Esper 3.2 documentation has a nice summary on how match_recognize compares to Esper well established select from patttern[ ... ] and followed-by EPL language features.
It is also great to see EsperTech on the Forrester Wave Complex Event Processing Platforms, Q3 2009: "Forrester evaluated nine complex event processing (CEP) platforms using 114 criteria (...). EsperTech, the only open source option, is also a Strong Performer."