What is the difference between Canon "L" lenses and non-L lenses?
According to Canon, L lenses contain all their best technologies like ultrasonic focusing motors, florite and aspherical lens elements for best optical performance, and are built to survive being used by the pro photographers. Many times this also means sealing against dust and humidity.
Let's take these two lenses:
- EF 70-300mm f/4-5.6 USM IS - consumer zoom, costs ~€450
- EF 70-300mm f/4-5.6 USM L IS - pro zoom, costs ~€1500
In this case, the L version is much better built, 50% heavier, with better image stabilizer, more sophisticated optical construction, weather sealing, ring-type USM with full-time manual (consumer version has just micro-USM motor). Optically the L version performs better than the non-L, but not as good as EF 70-200 f/4 L IS, and nowhere as good as €400 EF-S 60mm macro lens.
Additionally, the L telephoto lenses are white, which is meant to reduce chance its insides overheat on the sun as well as tell everyone around you have a Canon L lens.
Overall: L lens will be heavier, optically and mechanically better, and more expensive than similar non-L lens. This does not mean it will outperform everything else regardless of lens type.
Originally L simply stood for Fluor which is more sensitive to heat than glass which is why the lenses were painted white.
And not all the L lenses are white. Smaller ones (in term of focal length) are also black but keep the red ring (16-35 and 24-105 for example)