JDBC is a layer of abstraction on top of vendor-specific relational DB drivers. Without JDBC you would have to deal with peculiarities of a specific DB (not much fun). JDBC, however, is too low-level and entails a lot of boilerplate code.
JPA is a specification of an ORM (just an interface). It's useless without an implementation.
ORM is a kind of framework concerned with saving and retrieving objects to/from the relational DB. There are many ORMs out there with different levels of abstraction. Some of them require manually-written SQL.
Some of those ORMs implement JPA (Hibernate or EclipseLink, for example). Most of them are built on top of JDBC.
Such ORMs provide the maximum level of abstraction to the point you almost never have to write SQL queries. Some people love JPA-based ORMs (they reduce boilerplate), some hate (abstraction is leaky, specification is overly complex and there are lots of corner cases).
Java analogy:
class ORM extends JDBC implements JPA { }