From Fan History Wiki
A sock puppet is an additional identity created by a person for online use.
In practically all cases, a sock puppet pretends to be a separate person and is used deceptively to boost positive opinions of its creator by either complimenting him/her or insulting him/her to gain attention and encourage defense of the original. Because of this, the term "sock puppet" is considered negative and an insult.
Sock puppets are used to hide the identity of the original user. The creation of a new identity then acts as a "restart" button on discussions, or hides any biases that would be revealed by knowing the online identity of the original user.
They are used to speak on behalf of the original user. This spokesman (sometimes identified as a relative) can give information for the original user, such as updates on a non-existent incurable illness.
Sock puppets can also be used to mock an attack on the original user. This attack then garners sympathy and helps the original user gain attention, such as in the Msscribe wank in the Harry Potter fandom.
Role-playing and Pseudonyms
This is distinct from practices such as role-playing game players creating separate character identities, since these character identities are understood to be fictional identities played by real (sometimes anonymous) people. Sometimes short-term identities are also created for the purposes of a joke or meme and never address their real identity; generally, these are not considered sock puppets because they exist for a different purpose.
While fan fiction writers commonly have more than one user name, such as one for their erotic fiction and another for their gen fiction, these separate pseudonyms are not considered to be sock puppets. The use of separate names for separate communities is an accepted standard practice.
However, long before Ms scribe, John A Bristol infultrated fandom. According to the The Canadian Fancyclopedia, in 1938, Jack Bristol Speer created the sock pupper John A Bistol, and the hoax wasn't discovered until 1939:
Perhaps the first hoax fan. Bristol was actually Jack Speer. When Speer moved from one address to another in Washington D.C. in 1938, he gave out his new address as Bristol's, and had the post office forward to his new address any mail sent to Speer at the old address. In his own words: "By giving Bristol a full background of life, easing him in gradually, and taking great care to have him speak like a newcomer and use a style of writing and grammar quite different from his own, Speer got him generally accepted as a new fan." Donald Wollheim, who knew that Speer's middle name was Bristol, had his suspicions, but the hoax was not actually exposed until Nycon I, the 1st world convention, held in New York in 1939 (where Speer wore a John Bristol name tag-- surprisingly few fans noticed). Speer's 'Bristol' may have inspired Carr's 'Brandon'. (JS) & (DE)
In March 2008 in Kentucky, State Representative Tim Couch proposed legislation that would outlaw anonymous commenting.  While unlikely to pass, this type of legislation, if passed, would pose serious problems for sock puppeters in fandom.