Generalized anxiety disorder is a persistent feeling of excessive tension and worry. It's distinct from common fear in that it's future-oriented, persistent, intrusive, and general, whereas fear may be transient, present-focused, and specific, abating as soon as the perceived threat has passed. Alternatively, social anxiety disorder is this excessive tension and worry about normal social situations, which is less potent when not faced with triggering social interactions but may still persist with the mere thought of said interactions.
Other anxiety disorders include obsessive-compulsive disorder, which is focused around a compulsive activity, post-traumatic stress disorder, which is focused around a previous traumatic experience, phobias, which are focused on a particular irrational trigger of fear, and panic disorder, which is characterized by sudden and unexpected episodes of extreme panic, and usually don't last very long.
Anxiety is, to some extent, an excessive and poorly regulated fear response, which has its roots in dysfunctional brain and endocrine activity and thus could be diminished through foods and supplements. Foods and supplements that affect anxiety may do so through being or affecting the production of neurotransmitters and hormones involved in the fear response, affecting receptors, ion channels, synaptic plasticity, and more.
Anxiety is largely measured through a variety of questionnaires designed to assess the severity of symptoms. It may be assessed through behavioral means by trained observers, or by neuroimaging, but those are less common in research.