A sockpuppet (usually called a "sock") is an account on an Internet site that belongs to an already established user, often unbeknownst to other members of that site. Most commonly, sockpuppets are used to circumvent the blocking of an already established account or to exploit features of a website usually only accessible once (i.e. something that can only be given out once per account).
Editors are dissuaded from running sockpuppets and is cause for blocking. Some Administrators may use Sockpuppets occassionally or Single Purpose Accounts.
There is a subcategory of sockpuppetry called meatpuppetry, where a user recruits other real users to make accounts, possibly in order to sway opinion in discussions, or, particularly, in ballots. While this can be just as disruptive as sockpuppetry, it is harder to detect.