I used the virtual machines in virtualbox in a "headless" mode instead of a GUI mode. what are the advantages of using it in a headless mode?? by headless does it mean that the server doesnt have a keyboard or monitor attached or does it mean that no window will "pop up" , denoting that it is ON(or any other status), when a virtual machine is worked with? what exactly does it mean? pls reply...
The VirtualBox user manual talks about headless mode on page 32 under "1.12 Alternative front-ends"
VBoxHeadless is yet another front-end that produces no visible output on the host at all, but merely acts as a VRDP server. Now, even though the other graphical front-ends (VirtualBox and VBoxSDL) also have VRDP support builtin and can act as a VRDP server, this particular front-end requires no graphics support. This is useful, for example, if you want to host your virtual machines on a headless Linux server that has no X Window system installed. For details, see chapter 7.1.2, VBoxHeadless, the VRDP-only server, page 101.
One great advantage of running a VM in headless mode, at least on Unix/Linux, is that it is no more dependent of your graphic environment. That means you can start the VMs using nohup, connect to them through rdesktop or equivalent to start some jobs then quit your rdp client letting the VMs running on the background. You can log out without halting the VMs.