Wubi offers the possibility to install Ubuntu from within a running Windows session and boot it instead of Windows without changing the partition table or the boot loader.
At first Wubi, the Windows based Ubuntu installer, is nothing more than a tool to help users installing Ubuntu in parallel to Windows. Note: this does not involve any kind of virtual machine, the Ubuntu you end up with using Wubi boots instead of Windows.
However, there are many interesting technical details behind Wubi which make it special.
First of all: the boot loader which will be used will not be a Linux boot loader like Grub, but Windows’ native boot menu.
Second: instead of the usual need of an own partition for the Linux installation Wubi creates a single file on the Windows hard disk and embeds the Linux distribution inside. This avoids any hard disk partition table changes which can be quite difficult sometimes.
From the FAQ:
Wubi adds an entry to the Windows boot menu which allows you to run Linux. Ubuntu is installed within a file in the windows file system (c:\wubi\disks\system.virtual.disk), this file is seen by Linux as a real hard disk.
And of course, since this is Windows, everything comes with a nice GUI. It is a simple point-and-click procedure, much more easier than other ways.
Of course in these days were the available computing power allows you to easily run a virtual machine dual boot is not as attractive as it was some years ago. Also, if a user would now decide to ditch Windows he would be faced with a re-install. But still, since it is that easy it makes it one step easier for users to test and experiment with Linux.
There is also a Linux version of this tool available: Lubi. It looks like it does not feature a GUI yet, however it makes testing other distributions also much easier.