New to Typophile? Accounts are free, and easy to set up.
"Legibility" is based on the ease with which one letter can be told from the other. "Readability" is the ease with which the eye can absorb the message and move along the line.
--from ‘Types of Typefaces,’ by J. Ben Lieberman, 1967
Whether this distinction reflects an underlying difference in reality is controversial. For some of the controversy see: