Canon "EF" lenses are "full frame" (the image they project into the sensor body is large enough to completely cover a full-frame sensor... which of course means they also are more than large enough to cover a crop-sensor.) As such they work all Canon EOS bodies.
What you get when you go a notch higher into the "L" series lens is what Canon considers to be their top of the line quality. There are some non-L lenses which have optical quality that rivals the L series, but L series typically have more than merely really good optics. L series lenses that auto-focus always use USM type focusing motors, they are generally always weather-sealed (with a few exceptions -- you'll notice a rubber (probably silicone) gasket where the lens mates to the body flange.) They usually have more aperture blades and the aperture blades are rounded. Often an L series zoom will be able to maintain the maximum focal ratio all throughout the zoom range (not always though). The lenses generally have better physical body build. The list goes on.
L series lenses always have a red stripe around the end of the lens.
You can use any EF lens with a 5D Mk II -- it doesn't have to be an L series lens. You can use 3rd party lenses as long as are intended to work with "full frame" cameras (3rd party lenses won't say "EF" or "L" - those are Canon's nomenclatures.)
There are several speciality lenses which ARE full frame but are NOT marked as "EF" lenses. The "MP-E" 65mm macro is one. The line of tilt-shift lenses are "TS-E" (there's a 17, 24, 45, and 90mm) also don't say "EF" on them, but are all full-frame lenses. All of these work with every Canon EOS camera as well.
Basically you can use any lens Canon currently makes EXCEPT the lenses marked as "EF-S" -- those lenses only provide image circles large enough to cover the size of an APS-C crop-frame sensor. Also the rear-most element of an EF-S lens actually sits back into the sensor body slightly (which doesn't happen on EF lenses). They can get away with this on a crop-frame body because the smaller sensor means they can use a smaller mirror. On a full-frame body with a larger full-frame mirror, the mirror requires more clearance to swing up and out of the image path when you shoot. The mirror would hit the rear-most element on an EF-S lens. There's a very subtle design difference on the full-frame vs. crop-frame body cameras that Canon did to make sure you cannot accidentally mount an EF-S lens onto a full-frame body (So even if you buy the wrong type of lens, you'll discover you can't mount it on your camera. It won't fit.)
If you buy a lens that allegedly works with Canon EOS cameras, and you discover that it doesn't want to fit on your 5D Mk II... don't force it. That would be the indicator that you mistakenly purchased a lens intended only for use on APS-C crop-frame sensor bodies. You would want to return such a lens and make sure you buy a lens designed to work on full-frame bodies.
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