JPA is a specification that you implement in your data layer to perform db opertations, OR mappings and other required tasks.
Since it is just a specification, you need a tool to have it implemented. That tool can be either Hibernate, TopLink, iBatis, spring-data etc.
You don't necessarily require JPA if you are using Hibernate in your Data Layer. But if you use JPA specification for Hibernate, then it will make switching to other ORM tools like iBatis, TopLink easy in future, because the specification is common for others as well.
*(if you remember, you do import javax.persistence.*;
when you use annotations for OR mapping (like @Id, @Column, @GeneratedValue etc.) in Hibernate, that's where you are using JPA under Hibernate, you can use JPA's @Query & other features as well)