It's a combination of essentially FLIR and NVG's, although built into the aircraft and displayed in the HUD rather than worn as goggles.

FLIR, Forward Looking Infra Red, uses far-IR to see the heat of terrain, and essentially turns the night into day, at least for the field of view that it "sees."  NVG's use near-IR and magnified visible light.  The near-IR doesn't see terrain except to the extent that it's illuminated by something (moonlight, starlight), and far-IR can't do much to tell the difference between a light that's on or off (so you can see the runway edge, but you can't see the runway edge lights using FLIR, for instance).

The two systems have slightly differing capabilities and limitations; when both are fused together onto a display (i.e. a Heads-Up Display, or HUD), the pilot gets a pretty amazing picture of what is directly in front of him: the far IR shows him the runway edges, taxiways, pavement, buildings, obstructions,etc, and the near-IR makes lights (i.e. runway and taxiway lights) stand out.  In low visibility, the far-IR tends to be somewhat more degraded than the near-IR, whereas at night & with no moon, the near-IR shows the lights but not much else.  Together, the pilot can get the runway lights in sight sooner than he could unaided, and by the time he's landing he can see "everything" in the field of view of the HUD, not just the lights themselves.

Example videos:

 - [YouTube][1] 
 - [Vendor's Website][2]
 - [AvWeek][3]

When the videos get down close to the runway, you can pick out the ***glowing*** approach and runway lights, which is where EVS differs from pure FLIR.  With FLIR, you'd see (sort-of) the lights and any framework that they're mounted on, but they wouldn't particularly glow -- they might as well be off.  With EVS, the crew can fly a CAT I ILS and actually have the approach lights, runway, runway lights, etc all in sight at the 200' DH, even when the visibility is such that with the naked eye, none of that would be visible.  At major US airports, CAT III approaches are often available, but elsewhere they may be rare or nonexistent.  The EVS capability has the promise of allowing crews to successfully land out of a CAT I approach (i.e. at an airfield that only has Cat I ground equipment) even in CAT III weather.  And for a global carrier like FedEx, that's a capability worth having.

  [1]: https://www.youtube.com/watch?v=DR9lyAM2YNE
  [2]: http://www.elbitsystems-us.com/commercial-aviation/products/enhanced-flight-vision-system-evs/efvs-action-video-gallery
  [3]: http://aviationweek.com/business-aviation/video-flight-enhanced-vision-system-and-without