I condensed information from the user manual for you
Shared Networking (NAT)
In this mode your virtual machine can access the Internet while being invisible to other computers on the network (except your Macintosh computer). This mode is recommended because it doesn't require any additional configuring. Your virtual machine will share whatever network connection your computer uses at the moment.
This mode is preferred in the following cases:
your computer accesses the Internet via a modem or another non-Ethernet device,
as a mobile user, you access the Internet from Internet cafes, hotel rooms and on flights,
you want to access the Internet in a virtual machine but you are concerned about security,
you have problems with Bridged Ethernet mode.
If your virtual machine uses the Bridged Ethernet type of connection to the network, virtual machine appears on the network as a stand-alone computer with its own IP address and network name.