Like other VM software, VirtualBox has the concepts of NAT and Bridged networking for the VMs. In case you have only used the defaults so far and don't quite understand these concepts, NAT is network address translation. It's like your router at home does. The virtual machine gets a private IP address (maybe something like 192.168.1.10) that allows it to make connections to the outside world for things like surfing the web, checking email, etc. NAT networking does not allow connections to be initiated from the outside world into your VM though (for things like running a web server from your VM). Bridged networking creates a virtual bridge inside your kernel and then attaches the real network card and multiple virtual network cards to the virtual bridge to allow your host machine and the virtual machines to appear as machines on the real network (by real network I mean the externally addressable, non-private, network).

By default, VMs created with VirtualBox can access the internet through NAT networking without any extra effort on your part. In my workplace we often use VMs in our development process and we want these VMs to serve up web content and therefore to use Bridged networking.