Introduction
To quote the website:
VirtualBox is a powerful x86 and AMD64/Intel64 virtualization product for enterprise as well as home use. Not only is VirtualBox an extremely feature rich, high performance product for enterprise customers, it is also the only professional solution that is freely available as Open Source Software under the terms of the GNU General Public License (GPL) version 2.Installation
Download for your system, e.g. OS X hosts.
Open the dmg image and double click on VirtualBox.pkg; follow the instructions taking all defaults.
Use spotlight or launch pad to open VirtualBox
Download the Extension Pack; to install it, click Preferences in VirtualBox, then Extensions, and the +. Navigate to the downloads folder and select the package.