Tasks can have more states than just 'to do' and 'done'. While this is a perfectly fine default, some users want more states. I'm one of those. I've been using this in org-mode for years. See the manual here: http://orgmode.org/manual/TODO-extensions.html#TODO-extensions

In addition to the traditional 'todo' and 'done' states, I also use 'waiting', 'cancelled', and 'inprogress' to denote that I can't or won't work on something, or that I am actually working on something (and should finish it before taking on a new task).