The recommended way to install and maintain the Open Source (GPL v2) Virtual Box >= 4.0 in Ubuntu would be to add the following repository to your sources list:
deb http://download.virtualbox.org/virtualbox/debian <ubuntu-release> contrib
(replace <ubuntu-release by your Ubuntu version, e.g. oneiric). Then download and install the signature key:
wget -q http://download.virtualbox.org/virtualbox/debian/oracle_vbox.asc -O- | sudo apt-key add -
After updating your sources list with
sudo apt-get update
You can install Virtual Box via the Software Center or by typing
sudo apt-get install virtual-box-4.1
(replace -4.1 by the current Virtual Box release). If you don't want to compile the kernel driver every time your kernel updates it is also recommended to install dkms by:
sudo apt-get install dkms
To gain extended funtionality such as USB 2.0 support or support for Remote Desktop Protocol (RDP) you can make your Virtual Box installation close source (PUEL) by downloading the Oracle Virtual Box Extension Pack and installing this by double click or with the Virtual Box Manager GUI under File -> Preferences -> Extensions:

3D support (to run Unity), better desktop integration, changing the virtual screen geometry, and more can be achieved by installing the Guest Additions.