Shared-IP - One IP is shared with all connected users to that particular server. No ports can be forwarded or open for listening. Meaning you cannot have, for example ftp services in your connected device and access it when you are outside.
Dedicated-IP - Each connected user will be a ASSIGNED a dedicated IP address for his personal use for the duration of his active connection. While connected, if he has a service, for example ftp services running in the connected device, you can access the ftp while you are away from your device. You can open and forward any ports while connected to this type of server.