A task is an activity that needs to be accomplished within a defined period of time. Tasks are generally pieces of work that require effort and resources and have a concrete outcome or deliverable.

In theory a task can be of any size. Indeed, a project could be regarded as a very large task. However, the term task is normally used to refer a smaller piece of work.

Tasks take place over a period of time and usually consume resources. Projects are made up of tasks, sometimes grouped together into work packages. In practice, the term task can be applied to almost any project activity. Task durations can be long or short and there can be huge variations in costs and resource requirements.
The deliverables from many small tasks may be combined together to create the deliverable for a larger task. For example, a project manager may be allocated the task of producing a project plan. This task may involve obtaining information from a number of people. Each meeting could be regarded as a task in its own right. Some of the people the project manager meets with may also have to carry out tasks themselves in order to provide the required information.