Understand a JAVA code architecture, design

I am working recently in a JAVA EE project, where i have to check and understand existent code. the project architecture is Core,DAO,Manager,Web... In Core, we are supposed to write the POJO class that will later persist ! What i don't understand, why the developer is using interface first to define a User where he just expose some essential method used by other module (Web ...) then he define the interface implementation to use it in the core module. Is this a well known Design pattern ? if so What is it name ? thx