Sock puppetry is the creation of online identities for the purposes of self-promotion and the promotion of friends and for dissing or ignoring the competition and keeping the "unwashed" outside the clique.
For instance, those who create personae at AW and leave messages on a scribe friend's scroll, each time pretending to be a newbie and praising said scribe to the hilt for his or her kindness, have created sock puppets and are either consciously engaged in the promotion of a particular scribe over and above other scribes, or think they're doing their friend a favour.
Then there are those who create several personae who carry on conversations with each other, or use them to weigh in on discussions to score points; or to flood polls with their votes; or to leave messages on the scrolls of those whose attitude or behaviour they don't like; or even to attack someone outright.
These personae are known as sock puppets. This phenomenon is not by any means exclusive to AW. For a good example of executive sock puppetry, see the New York Times article, The Hand That Controls the Sock Puppet Could Get Slapped.
I find sock puppetry fascinating on so many levels, but especially the peculiar "shadow" selves who tiptoe around, thinking that no one will see them or catch on. It's got a child-like quality to it, like little kids who think they're pulling the wool over their parents' and teachers' eyes when they play dress-up, or like the "man behind the curtain" in the Wizard of Oz.
A word of caution:
There are a number of reasons besides sock puppetry for creating several personae at AW, such as academic study, exploring different cultures and roleplay. There are exceptions even in these areas, however, such as, the creation of personae for the express purpose of disrupting a roleplay game or undermining its leader(s), promoting an agenda, or taking sides in a dispute.
Other intersting articles
Sock Puppet Sinks Pundit
Forum Ethics and Official "Sock Puppets"
Internet's 'sock puppets' are threatened species