Don't worry if you didn't know, you wouldn't be the first! The difference is in the alcohol content and the U.S. Alcohol and Tobacco Tax and Trade Bureau (TTB) provides an official definition.
Shown below are the rules for labeling malt beverages from 27 CFR § 7.65 Alcohol Content parts (e) and (f):
Non-alcoholic. The term “non-alcoholic” may be used on labels of malt beverages only if the statement “contains less than 0.5 percent (or .5%) alcohol by volume” appears immediately adjacent to it, in readily legible printing, and on a completely contrasting background. No tolerances are permitted for malt beverages labeled as “non-alcoholic” and containing less than 0.5 percent alcohol by volume. A malt beverage may not be labeled with an alcohol content of 0.0 percent alcohol by volume, unless it is also labeled as “alcohol free” in accordance with paragraph (f) of this section, and contains no alcohol.
Alcohol free. The term “alcohol free” may be used only on malt beverages containing no alcohol. No tolerances are permitted for “alcohol free” malt beverages.
In fact, when beers are submitted to the TTB to be approved as "alcohol-free", like Ceria's beers, the brewery must send in a sample to the TTB to be analyzed to confirm 0.0% ABV.
Whether you're sipping AF or NA beers, now you know the difference. And that's great because an informed drinker is a happy drinker!