This skeletal information refers to Lightning 1.0b1 running on Windows XP. Behavior of the UI of other Lightning versions and of Sunbird is different in important details.
Lightning is installed as a plug-in to either Thunderbird or Eudora. If the plug in is properly installed, there is a new toolbar menu called Events and Tasks. The calendar and task list will not appear in the T'bird or Eudora window until you select either "Calendar" or "Tasks" from this menu.
Tasks are presented either as an entire window (not very useful, but available) or as a side-bar to the right of the main calendar pane. The default task list shows all open tasks, irrespective of start or complete date. The task list ordering is selectable by column header (with the column headers enabled/disabled using the pick-list icon in the header bar). There do not appear to be any list filters other than "hide completed tasks." Therefore, all future tasks are presented.
Tasks by default are created without start or end dates, with no category, status, repetition, or reminder settings. Tasks are associated with one calendar only, like Events.
Tasks when presented in list view are color coded: black when the task is before the start date, green when it is after the start date, and red when it is after the end date.
There's a bug in the task edit pop-up: if you change a start or end date but do not change the time of day for that item, the change will not be saved. All you have to do is change the time by 1 minute, so it's an irritant with a work-around (read: won't be fixed in your lifetime)