VirtualBox is a family of powerful x86 virtualization products 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). See "About VirtualBox" for an introduction.
Wine-doors is an application designed to make installing windows software on Linux, Solaris or other Unix systems easier. Wine-doors is essentially a package management tool for windows software on Linux systems. Most Linux desktop users are familiar with package management style application delivery, so it made sense to apply this model to windows software.