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Free sugars are defined by the World Health Organization and the UN Food and Agriculture Organization in multiple reports as "all monosaccharides and disaccharides added to foods by the manufacturer, cook, or consumer, plus sugars naturally present in honey, syrups, and fruit juices". The term is used to distinguish between the sugars that are naturally present in fully unrefined carbohydrates such as brown rice, wholewheat pasta, fruit, etc. and those sugars (or carbohydrates) that have been, to some extent, refined (normally by humans but sometimes by animals, such as the sugars in honey). They are referred to as "sugars" since they cover multiple chemical forms, including sucrose, glucose, fructose, dextrose, etc.
In 2015, the World Health Organization (WHO) published a new guideline on sugars intake for adults and children, as a result of an extensive review of the available scientific evidence by a multidisciplinary group of experts. The guideline recommends that both adults and children reduce the intake of free sugars to less than 10% of total energy intake. A reduction to below 5% of total energy intake brings additional health benefits, especially with regards to dental caries. 
The principal definition of free sugars was to split the term "carbohydrate" into elements that relate more directly to the impact on health rather than a chemical definition, and followed on from meta-studies relating to chronic disease, obesity, and dental decay. It also led to the WHO and FAO to publish a revised food pyramid that splits up the classic food groups into more health-directed groups, which appears, as yet, to have had little impact on the food pyramids in use around the world.
The inclusion of such a definition caused issues for the WHO with sugar companies, who attempted to get the US government to remove funding from the WHO for suggesting that consumption of free sugars within the food pyramid should only amount to a maximum of 10% of the total energy intake, and that there should be no minimum (i.e. there is no requirement for any free sugars in the human diet) on the basis that the report did not take into account the evidence supplied by the sugar industry. The report in question specifically includes references to the evidence, but was unable to use it for a health basis, as the studies did not offer effective evidence of an impact on health, and referred to such studies as "limited".