VirtualBox 4 got a cleaned-up user interface, expanded virtual-machine configuration
support and an internal reorganization in order to carry on the battle with VMware Workstation and Citrix XenDesktop. In the second release since Oracle acquired Sun, VirtualBox gained support for OVA (Open Virtualization Appliance), which is a single, compressed file of all the elements needed for a virtual appliance that conforms to the OVF (Open Virtual Machine Format)standard. Along with internal reordering that combined VirtualBox base functions to more easily enable extensions, the product rearranged the VM file structure by putting the virtual disk and configuration files for virtual machines into a single directory. The file layout changes don't affect VMs created in previous versions of VirtualBox.