Difference between "Guest Network" and "Isolated Network" under Advanced Networking

[Sachchidanand Vaidya]

Hi,
    Under "Networks" Tab in UI for Admin user, Cloudstack provides 2 options to 
create networks
    1) Add guest network 2) Add Isolated Network.
     - What is the difference between these 2 networks.

    Also, I created and Enabled a new "Network Offering". But when I create an 
Isolated Network,
    I don't see the newly created Network offering in the drop-down menu. Are 
there additional steps
   to be followed to make new n/w offering visible during Virtual Network 
creation.

Thanks,
Sachin