This information assumes you have Windows 7 as the operating system on your current machine - not Windows XP.  If you're running XP then your only options truely are dual-boot or a virtual machine. You must also download Windows 8 evaluation edition from the trial site.

Windows 7 has the ability to "boot from a VHD".  A VHD is a Microsoft's virtual hard drive format.  However, in this case it is being used simply as a disk for the REAL hardware of the machine.  VHDs can be created directly in Disk Manager on Windows 7.  Disk manager is normally only concerned with managing your physical disk drives - but it has been extended to allow VHDs to be created and managed just like real hard drives.  Essentially you are creating a "Virtual Partition" by using a VHD in this way.

So why would you want to bother with this method of dual booting?

It has many advantages of virtual hard disks (advantages over using a real partition):

• No need to re-partition (risky and time consuming) an existing drive to give Win8 a dedicated boot partition.
• No need to loose all the disk space of the Win8 Partition on your real machine.
• Using a "Dynamically Expanding" (growable) VHD your "virtual partition" only consumes the amount of space actually being used by the OS inside of it.
• When you are done with it, simply delete it, no repartitioning your machine back to it's previous state.
• You can mount the VHD in Windows 7 and perform data transfer if necessary.
• Super easy backup of the entire machine (VHD).
• Possible sharing of the bootable VHD between hardware identical machines.

It also has advantages of using real partition (advantages over using a Virtual Machine):

• The loaded OS runs on your real hardware - giving it a big performance edge over a Virtual Machine
• You must load (and therefore test) your real hardware drivers - revealing whether all your hardware is satisfactorially supported with no risk to your operational machine.

There is one change made to your real machine when you load Windows 8 in this manner.  The Windows 8 boot manager is loaded on your real hard drive so that it can service boot requests and present boot options. This doesn't really present any problems as it will list any other installs you have currently in your boot configuration.  Also, if you delete the VHD, the Windows 8 boot manager will still be operational.

Like any installation onto a real machine, you must use the Windows 8 install media externally - either a USB flash drive or a DVD.  (Installing on a full virtual machine can be done directly with an ISO.)

Also, since you are not running an actual virtual machine, you do not have the ability to use snapshots.

There are two how to articles that keep the steps simple and use screenshots appropriately.

This one is simple and requires no external files, but does have some advanced steps during the first boot from install media.  It also contains steps for creating a bootable USB flash drive from an ISO: http://www.zdnet.com/blog/bott/how-to-use-a-vhd-to-dual-boot-windows-8-on-a-windows-7-pc/4847

This next one simplifies the process during the first boot from the install media, but requires a downloadable powershell script and assumes DVD media: http://www.howtogeek.com/107535/how-to-seamlessly-dual-boot-windows-7-and-windows-8-the-easy-way/