Net driver (a.k.a. thin driver) allows clients to access the remote databases without any client side installation. In most cases, it uses TCP/IP protocol to go over the wire. The db2 server should be running "JDBC Applet Server" to accept such requests.
App driver (a.k.a. thick driver) requires client installation of DB2. Generally it uses proprietary protocol to talk to the remote database. I believe you have to catalog the remote database before you can connect to it.