A flack is a person—especially a press agent or publicist—who talks up his or her employer and deflects criticism. Flak (usually a mass noun) refers to (1) antiaircraft artillery, and (2) excessive or abusive criticism (the second definition derives metaphorically from the first).
RIM has taken a lot of flak from CNET and others for its decision to pair the BlackBerry PlayBook tablet with a BlackBerry smartphone. [CNET]
He promptly became a political flack. [Gawker]
Brown budget plan draws flak from right, left [LA Times]
So far, Carney’s been successful in making the transition from journalist to administration flack. [Yahoo! News]