According to Problems with Java EE SDK, Java EE SDK is the Java EE interfaces with RI concrete implementation. (which happens to be GlassFish).
I know that I still need an Application Server to deploy my apps, So why I need this SDK?
Is it to reference in my API that depends on the Java EE APIs? (example in GWT they include interfaces of Servets) So why the RI included?
The downloadable Java EE SDK from oracle.com includes not only the Java EE APIs, but also implementations of these APIs (Glassfish application server, the reference implementation). This SDK also includes tutorials, code samples, and documentation. The Java EE 6 SDK is a complete application server and development kit. This download may be helpful to provide a full environment for developing and deploying Java EE applications.
You do not technically need this particular downloadable SDK to develop and deploy Java EE applications, however. To develop Java EE applications, you only need the Java EE APIs, which can be obtained from the Java EE SDK or from a number of other sources (e.g. java.net maven repository, or from other application servers or repositories). The APIs alone (i.e. the JAR files) will enable you to build your EE applications against the Java EE specification. Then, you can download any Java EE compatible application server (Glassfish, WAS, JBoss, etc, etc.) and deploy your applications.
the sdk comes with Glassfish; which is the reference implementation for application servers.
