IS ll is a newer & better (IS) Image Stabilizing system, which helps cancel out camera shake
EF = a lens that will work on any EOS camera with a full frame, APS-H or APS-C sensor.
EF-S = a lens that will only work on an EOS camera with an APS-C sensor.
IS = image stabilization - an electro mechanical system in the lens that counteracts camera shake.
IS II = an improved version of IS.
USM = Ultrasonic Motor - refers to the focusing motor in the lens. USM lenses focus faster.
L = Canon's designation for "pro" quality lenses. L lenses have excellent optical quality, are mechanically rugged and many are weather sealed.
It is difficult to agree with the statement that "an L lens will be better than a non-L lens" unless you define "better" precisely. There are several non-L lenses that are universaly recognized as being of L quality optically including the EF-S 17-55mm f2.8, the EF-S 15-85mm f3.5-5.6 and the 100mm f2.8 macro. It is interesting to note that Canon has never given the L designation to any EF-S lens. Some L lenses are weather sealed but many are not so you cannot make a general statement that L lenses are better in this regard. The closest you can come to making a generaization about L lenses is that they are more rugged mechanically than most other lenses.
I have to agree with that last message. L's are usually considered more rugged & of better quality IQ wise compared to a similar consumer grade lens, but that doesn't always mean we could see an IQ difference short of in a lab test.