The mean camber line of an airfoil is the line from the leading edge to the trailing edge, of which each point lies exactly halfway between the upper and lower surfaces. For an airfoil with zero camber (i.e., the wing is symmetric about the chord line), the mean camber line is straight and corresponds to the chord line. In other cases, the mean camber line is curved.
upper surface
_____
-- --------_______
/ _----________ -----...
(------------------------------->
\ _____--------_______-----'''
--
lower surface
The chord line is the dotted line through the center; the mean camber line is the dotted and flat line just above it, and the dotted line where only that exists.
Sorry for the horrible ascii art; for a better picture, see http://www.grc.nasa.gov/WWW/K-12/airplane/geom.html.
Indirectly inspired by a look at my nodeshells. | 
