A Task is something that you and your team need to accomplish in a particular pod. They are single items within a task list.
A Milestone is a marker or a goal that you want to achieve for a list of tasks. A task list can be associated to a milestone and the progress of a milestone is tracked as you complete tasks in your task list.
An Event is only associated to your calendar and not to a pod. Events are things like seminars, meetings, special events, etc.