OpenJDK is the short form of Open Java Development Kit similar to Oracle's Java Development Kit used to develop Java Applications. Main difference between Oracle JDK and OpenJDK is that OpenJDK is free for all while Oracle Java Development Kit or Oracle JDK requires commercial license.
OpenJDK is the open source implementation of the Java Platform Standard Edition and is free to use with General Public License Version 2 ; GPLv2. OpenJDK 7 is the reference implementation of the Java SE 7 Specification. For more details you can refer the official Java SE 7 specification from Java Community Process here.