Question: I) I know that virtual-photons are known to be the force-carriers for the Electromagnetic force, and that they are called "virtual" because the Energy-Time-inequality version of the Heisenberg Uncertainty Principle allows for particles that are high enough energy that they are very difficult to observe (because higher energy means a smaller possible time-scale for observation). 

But I also know that photons are the quanta of EM radiation; i.e. they emitted from atoms at some point in space, and absorbed at other points in space as a means of transmitting radiation energy.

My question is this: are the photons that act as the force carrier of the Electromagnetic force the "same" photons (i.e. the exact same particle) as the photons that act as the quanta of EM radiation? 

Is it just that the photons emitted as virtual particles have high enough energy that they act as a force carrier? If so, what causes charged particles to emit photons of such high energy?

II) As an add-on question: I'm being introduced loosely to Electro-weak Unification and the idea that at high enough energy, the EM- and Weak forces become indistinguishable from one another (and, I believe, that the difference between the EM-force and the Weak force, at low energy, is that the W and Z bosons that mediate the Weak force are massive, and therefor act at low range, whereas photons are massless and therefor act at long ranges). And subsequently, that the Higgs Boson helps to explain what gives W and Z bosons mass. 

But what is the difference between the W and Z bosons and the photon that makes them interact with the Higgs mechanism, and the photon remain unaffected?

I hope these questions make sense.

Answer: 
There is only one kind of photon.

Indeed, when we describe elementary interactions between two electrons for example, we call the photon "virtual" as opposed to a physical photon that might exist outside of this process.

Still, these are the same particles, i.e. excitations of the same fundamental field, as the photons that make up light for example.

Again, *virtual* photons can only appear in the context of a direct interaction between charged particles, while *real* photons are the electromagnetic waves send out e.g. by excited atoms. Macroscopic (constant) electric and magnetic fields are [coherent states](http://en.wikipedia.org/wiki/Coherent_states) of virtual photons.

Regarding the electroweak unification you seem to have a misconception. In the unified theory there is no electromagnetism any more, but only the electroweak force, which has four force carriers: The $W^\pm, W^0$ and $B$.

The Higgs field couples to all of those, giving mass to the $W^\pm$ and to a linear combination of $W^0$ and $B$, which we call $Z = \cos\left(\theta\_W\right) W^0 + \sin\left(\theta\_W\right) B$, while the orthogonal linear combination $\gamma = -\sin\left(\theta\_W\right)W^0 + \cos(\theta\_W)B$ remains massless.

So the photon is *defined* as the boson that remains massless after electroweak symmetry breaking.
A major difference between real and virtual photons is that virtual particles are not required to have energy and momentum [on the "mass shell"](https://en.wikipedia.org/wiki/On_shell_and_off_shell). That is, virtual photons may have $E^2-p^2 \neq m^2$, while real photons must obey $E^2-p^2=m^2=0$.

My memory disagrees with Neuneck (v1): I think that a coherent superposition of real photons is a laser, while static electric and magnetic fields are composed of virtual photons. For instance, consider the field between two point charges with opposite sign. Real photons moving along the path between the two charges can produce only transverse electric fields, but the static field is entirely longitudinal. A virtual photon with $m\neq0$, however, isn't restricted to the two transverse polarization states.

Neuneck's description of the weak mixing angle is spot-on. I'd add that you *do* recover a "pure" electroweak force in the limit where the energy $E$ involved in the interaction is much larger than the $W$ and $Z$ masses, $E\gg m\_W$. In that case you can use the approximation that all four electroweak bosons $W^\pm,Z^0,\gamma$ have the "same" mass $m\approx0$.