As others have mentioned, EF stands for electronic focus, and the "-S" in EF-S stands for short backfocus. Both types of lenses are compatible with current production EOS bodies that have APS-C size sensors (what Nikon calls "DX"). EF-S lenses, however, cannot be used on bodies that have larger sensors. The following recent production EOS bodies are NOT compatible with EF-S lenses:


1D Mark III, 1Ds Mark III, 1D Mark IV, 5D, 5D Mark II.


EF-S lenses are physically incapable of being mounted on these bodies, unlike Nikon DX lenses, which can still be mounted on FX bodies but the periphery of the frame will be dark (because the DX lens projects an image circle that is too small to cover the entire FX frame).


"L" designates the professional line of Canon lenses. The closest they get to explaining what makes a lens "L" is found in EF Lens Work III, Chapter 2:


The bright red line engraved on the lens barrel. And an L for “luxury.” The Canon EF lens L series possesses a level of quality sufficiently high to be called professional, designed to include groundbreaking image performance, outstanding operability, and resistance to weather and aging. “L.” This name is reserved only for those few lenses that can meet stringent standards of performance, using fluorite (an artificial crystal), a ground and polished aspherical surface, UD, super UD lenses, or other special optical materials. Optical design without compromise together with optical theory and precision engineering technologies that are as steeped in tradition as they are cutting edge. And the result of our relentless pursuit of these ideals is the L series of Canon EF lenses.

Please note that no EF-S lenses are ever given the L designation, despite some of them being of comparable optical quality.


Regarding a wide-angle (< 70mm), f/2.8 or faster, image-stabilized, weather-sealed lens, there is no such lens available for the Canon system at present. The following lenses are close:
  1. EF-S 17-55/2.8 IS (not weather-sealed)
  2. EF 24-105/4L IS (one stop slower)
  3. EF 24-70/2.8L (not image-stabilized)



However, because weather-sealing is not especially robust for extending zooms (as opposed to weather-sealed primes and internal-focus zooms), I would not consider such sealing to be particularly essential. If you are willing to forgo sealing, then the EF-S 17-55/2.8 IS will be the ideal choice for your purposes on the 7D. Please note that Nikon also does not offer a lens that satisfies your four criteria. Indeed, Nikon offers no lens that satisfies the first three criteria--weathersealed or no, there is no lens wider than 70mm, f/2.8 or faster, with VR, in either FX or DX format.


While you did not ask for this information, there are no EF primes shorter than 100mm that are image stabilized. The shortest IS prime is the EF 100/2.8L macro IS, followed by the EF 200/2L IS and the supertelephoto series.