'L' lens are aimed at the professional shooter, made to be a bit more rugged in fabrication of the lens barrel and theoretically better able to withstand abuse (but on POTN we see L lenses breaking in half nonetheless), often with more weatherproofing than non-L, often colored off-white in long FL designs in order to resist temperature induced issues when working out in the bright sun for prolonged times.
Usually ASSUMED by many that 'L' means 'better IQ' as well, when in fact that there are some L lenses which are quite ordinary in performance (100-400 is one example), and where lenses for APS-C format are sometimes far superior in IQ but lack the L designation simply because they would not cover a 24x36mm sensor size adequately.
