Avoidant Personality Disorder is one of the more common personality disorders. Experts estimate that about 5% to 7% of people on the community can be diagnosed with Avoidant Personality Disorder (APD).
Based on the definition in DSM IV-TR, the main characteristic of Avoidant Personality Disorder is to think of oneself as inadequate, flawed and inferior to others.  People with APD tend to believe others don’t like them and are afraid that others will criticize or ridicule them.  This fear of disapproval, rejection, and criticism often causes people with Avoidant Personality Disorder to stay away from social interactions, and to avoid work or school activities that involve getting into contact with other people.  This frequently results in missing out on social and professional networking opportunities, and leads people with APD to have a rather small social circle in which they only interact with people of whom they are sure that they are liked. People with APD are extremely sensitive to rejection and criticism and they usually don’t like trying out new activities that might put them at risk of being embarrassed or ridiculed.
Social Phobia can look very similar to Avoidant Personality Disorder. Social Phobia has to do with being extremely anxious in social situations. Examples would be fear of public speaking, or fear of eating, drinking or writing in front of other people. It could be fear of addressing authority figures, fear of attending parties, or fear of initiating conversations. The fear is mostly about being embarrassed, or of others recognizing that they are anxious. When a person with social phobia finds themselves in their feared situation, they develop intense anxiety with some really strong physical symptoms, such as intense heartbeat, breaking out in a sweat, or hands shaking and shortness of breath. Taken to an extreme, these physical symptoms of anxiety can develop into a full-blown panic attack.
Avoidant Personality Disorder very often overlaps with other personality disorders, for instance, schizoid or dependent personality disorder, and with anxiety disorders, such as panic disorder or obsessive-compulsive disorder. What has been widely studied is the overlap between Avoidant Personality Disorder and Social Phobia. Different studies looking at how many people with Avoidant Personality Disorder meet criteria for Social Phobia, according to Lynn Alden and colleagues, range from 42% to 100%.
Alden and colleagues in their review paper on this topic make a good point: If you look at the definitions of these two conditions given in the Diagnostic and Statistical Manual for Mental Disorders, the overlap between the two conditions makes sense: The definition of Social Phobia focuses on performance situations, but it includes difficulty with dating and friendships. These latter difficulties are a core feature of Avoidant Personality Disorder. It is not surprising, then, that there would be significant overlap between two conditions that are defined in similar ways. This is the reason that some experts believe that the two disorders reflect the same underlying problem, with Avoidant Personality Disorder merely constituting a more severe and persistent form of Social Phobia.  This is why some believe that the two disorders should be combined for future incarnations of the DSM into a category of Social Anxiety Disorder.
Source:Â http://www.mentalhelp.net/poc/view_doc.php?type=doc&id=30080&w=11&cn=8