Websphere server 6.1 ND not starting because of nodeagent issue. FilenotfoundException

First one basic question:

What is the concept of cell, node, nodeagent etc. I understand that it is for network deployment/load balancing of application servers. But its not clear.

Second my problem with starting the server:

I created a websphere server profile as Cell (deployment manager and a federated application server) using Profile Management tool. I see AppSrv02 and Dmgr02 folders got created.
Next I wanted to use this server inside RAD (7.5). I created a new server, chose this profile name (AppSrv02), gave a server name (WebSphere Application Server v6.1 at localhost), a application server name  (atgServer).
I get this error when starting the server:

 E   ADMU0111E: Program exiting with error: java.io.FileNotFoundException: C:\Program Files\IBM\WebSphere\AppServer\profiles\AppSrv02\config\cells\malli1Cell02\nodes\malli1Node02\servers\atgServer\server.xml (The system cannot find the path specified.)
      at java.io.FileInputStream.<init>(FileInputStream.java:135)
        ........

This is what I have - C:\Program Files\IBM\WebSphere\AppServer\profiles\AppSrv02\config\cells\malli1Cell02\nodes\malli1Node02\servers\nodeagent
I have a nodeagent directory but the server startup is looking for atgServer folder (atgServer is the server name).

Any help appreciated!