Essentially, someone who asks continual questions and who seems unwilling to do any work, research, or to put any effort into learning on their own. Overview is here:

http://meta.stackexchange.com/questions/19665/the-help-vampi...

A common variation is someone who asks a reasonable question, gets an answer, then repeatedly modifies/extends the original question, or who asks a long series of follow up questions n comments. This variation is described here:

http://meta.stackoverflow.com/a/258279/1441122