What I perceive as a Virtual Machine is that it simply allows to load another Operating System side-by-side on its own sandboxed environment.
It is limited to the same hardware architecture as the host Operating System.
With Intel VT (and similar), one can run the 64-bit version of an Operating System on a 32-bit host.
Examples of this is are VirtualBox and VMWare.
But QEMU is a Hardware Emulator: it emulates an entire computer and (may) emulate an entire architecture with every single piece emulated.
This is not limited to the same architecture.
We can run the ARM version of Linux on QEMU running on a x64 CPU.
The Android SDK uses QEMU to emulate tablets and phones for testing.
Posing this, can we say that QEMU is a virtual machine?
This generated a lot of discussion in the comment on the question "How to prevent network administrators from accessing USB drive" on another website, in which there was a 'dispute' that an Hardware Emulator is still a Virtual Machine.