Q:

JPA where can I found some basic explanations

There are many resources where you can found information. Most of them highlights only some aspects. If you want to see overall information you should read a book.
Intention of this question is provide you with some light-weight article, that nevertheless covers all basics that you need to get started.

A:

Understanding JPA, Part 1: The object-oriented paradigm of data persistence
http://www.javaworld.com/javaworld/jw-01-2008/jw-01-jpa1.html
Understanding JPA, Part 2: Relationships the JPA way
http://www.javaworld.com/javaworld/jw-01-2008/jw-01-jpa2.html 

