Worldcon, or more formally the World Science Fiction Convention, the annual convention of the World Science Fiction Society (WSFS), is a science fiction convention. It has been held each year since 1939 (except for the years 1942 to 1945, during World War II). The members of each Worldcon are the members of WSFS, and vote both to select the site of the Worldcon two years later, and to select the winners of the annual Hugo Awards, which are presented at each convention.
I've never personally attended a WorldCon before, but as it's one of the largest science fiction literature conventions of the year, and takes place in a different city each year, I feel that I must attend it when it's right here in my city. If you're planning on attending, RSVP and we can work out times and places to meet in the comments.