The U.S. Food and Drug Administration (FDA) said in August 2013 that it would define "gluten-free" as containing less than 20 parts per million (ppm) of gluten.
Manufacturers will be legally allowed to label a food "gluten-free" even if that food contains an ingredient that's made from wheat, barley or rye (or a crossbreed from those grains) as long as it has been processed to remove the gluten down to below that 20ppm level.
As of the day the new rules were announced, the FDA said that most manufacturers 95% or more already were following the rules. However, the FDA gave companies a year to come into compliance.
The FDA first proposed rules to govern gluten-free labeling in 2007. In August 2011, the agency reopened the public comment period on the regulations and announced plans to resume work on the regulations, and in February 2013, the FDA wrapped up its work on the gluten-free regulations and sent the final version on for further regulatory review.
The Food Allergen Labeling and Consumer Protection Act of 2004 requires manufacturers to disclose on food labels the presence of eight major allergens (including wheat). Those disclosures began in 2006. The law also requires the FDA to establish rules for gluten-free labeling.
The agency issued proposed rules in 2007 that defined "gluten-free" as containing less than 20ppm.
The labels will be voluntary, not mandatory manufacturers that wish to cater to gluten-free consumers could add the labels to their packaging, but they wouldn't be required to do so. The labels won't eliminate the need for us to learn to identify gluten on food labels, either, since the FDA's rules won't require manufacturers to disclose gluten-containing ingredients.
However, if you're particularly sensitive to trace amounts of gluten, remember that the FDA's definition still allows tiny amounts of gluten in gluten-free foods, and it's still possible to get gluten symptoms from labeled gluten-free foods.