While every individual is unique, there are certain patterns of problems that seem to show up over and over again for many different people. Over time, mental health professionals have observed these patterns and assigned names to distinguish one from another. They refer to each of these common patterns as a "disorder" to emphasize that it is a change or a stepping-away from what is "ordinary". These disorders have been organized into different categories like "Mood Disorders", "Anxiety Disorders", or "Eating Disorders" and compiled in a useful book called the Diagnostic and Statistical Manual, which is currently in its fourth edition (DSM-IV).
Anxiety disorders can be seen as existing on a continuum with less extreme emotional difficulties. Each of the disorders described here involves anxiety that is more intense, lasts longer, and interferes with daily functioning more than anxiety that would not be considered a "disorder". Even if you do not meet all of the criteria for a given disorder, you may find that anxiety follows a similar pattern in your life, only to a lesser degree than described in the DSM-IV.