question about web dynpro

i am a beginner and the web dynpro is for java.

there is many type projects in the web Dynpro "new project winzard".

i want know when to use "web dynpro project" and when to use "portal application project",because you know,the applications of web dynpro project can be added into the portal.

i hava a quetion about connecting to the datasource in the "web dynpro project".i hava tried the follow way:

"jdbc/aliasname","jdbc/name","aliasname","java:com/env/jdbc/aliasname","java:com/env/jdbc/name","java:com/env/aliasname".i can't get the connection and i hava configure the jdbc connector using the j2ee admin tools and give it one aliasname .