In layman's terms:
JDBC is a standard for connecting to a DB directly and running SQL against it - e.g SELECT * FROM USERS, etc. Data sets can be returned which you can handle in your app, and you can do all the usual things like INSERT, DELETE, run stored procedures, etc.
JPA is a standard for Object Relational Mapping. This is a technology which allows you to map between objects in code and database tables. This can "hide" the SQL from the developer so that all they deal with are Java classes, and the provider allows you to save them and load them magically