Both mood disorders and anxiety disorders occupy a continuum of psychological states and conditions, ranging from more healthy and effective, to less healthy and less effective. To understand the overlap between the two, we first have to know the difference between emotion, mood, and behavior.
In psychology, emotions are understood as temporary internal states (feelings), due to specific inputs. For example, we may feel happy when we learn of a job promotion, or sad when we learn our pet dog has died. Emotions are impermanent and fleeting; they can change when we are exposed to new or different input.
Moods on the other hand, are sustained emotions that last for days, weeks, or longer. When news of a pet's death really starts to sink in, we may experience a general mood of sadness that can last months. This mood of sadness is sustained, even though we may experience other input that would usually make us feel happy. Moods, unlike emotions, are not easily changeable.
Mood disorders describe mood states that are so troublesome, unpredictable, or overwhelming as to interfere with our ability to lead healthy productive lives. For example, a persona with a depressed mood may not have the energy to go to work, interact with family, or take care of basic needs such as food and hygiene. In this case, one's mood has become so entrenched and overwhelming that one is stuck in it.
Anxiety disorders have to do with how we respond to various moods or events. When a person with an anxiety disorder hears bad news, that person may become upset, or fall into a low mood. However, in addition to becoming upset, a person with an anxiety disorder also reacts to bad news with a patterned set of behaviors. This set of unhealthy behaviors is what is identified in psychology as anxiety. Anxiety behaviors include: withdrawal, extreme reaction, panic, somatic responses (such as muscle spasms or immobility), loss of appetite, and excessive use of drugs and alcohol. Anxiety disorders, while usually encompassing low, depressed, or anxious mood, refer more specifically to a set of behaviors, not only one's internal emotional state or mood.