What is the difference between an epic, theme, story & task?

What is the difference between an epic, theme, story & task? When should they be used?

The event will be held in the upstairs room at The Plumbers Arms. The sessions are informal and we are happy for you to eat the pub grub / drink during the meetup (the pies are pretty nice).

The pub's site gives directions: http://www.taylor-walker.co.uk/pub/plumbers-arms-belgravia/c0685/

Here's their map:

http://photos3.meetupstatic.com/photos/event/f/c/0/event_149404032.jpeg