What is an anxiety disorder?
Anxiety disorders differ from occasional anxiety, as anxiety disorders involve more than temporary worry/fear.
Occasional anxiety is something everyone deals with, and is a normal part of life. Such as having some before taking a test or making an important life decision.
An Anxiety Disorder (I'll use Generalized Anxiety Disorder as an example) display excessive anxiety or worry for months to years, and is characterized by persistent, excessive, and unrealistic worry about everyday things. Often they anticipate disaster.