Don't be confused by my use of the terms "APD", "Social Phobia", "Social Anxiety" or even "shyness". They are all such similar conditions that they're pretty much interchangable. If they must be distiguished, it would only be by degree. Avoidant Personality Disorder is probably the most extreme form of Generalized Social Phobia, which is often simply referred to as simply social phobia or social anxiety. As for the very non-technical term "shyness", social anxiety/social phobia of the generalized type is basically shyness taken to such a degree that it subjectively causes a person a great deal of stress and/or interferes with functioning, such as being able to get or hold down a job.
People with APD frequently suffer from other mental disorders as well, most frequently anxiety and depression, which is understandable considering their difficulties in social situations and their anxiety about possible rejection. Depression frequently results from lacking a good support group. This personality disorder most frequently overlaps with Dependent personality disorder, but can also be found with Borderline and Schizotypal as well.