A lot of people make the mistake of confusing task and activity to be the same thing, which is understandable as the two words are being used interchangeably increasingly.
This, however, should not be allowed to change the fact that the two words actually have different meanings and therefore different usage.
In order to differentiate between task and activity, the first thing that a person should know is their meanings. The former basically refers to work that needs to be done or undertaken, whereas the latter simply refers to work, activity or an event that is currently active.
Just by going through the definition, the difference between the two words becomes obvious. However, the difference between the two words can be further underlined by looking at a few other things, such as their nature, presence and absence of authority, requirement of energy, involvement of movement, etc.
Task can also be referred to as an assignment, which is given or imposed on a person by someone through authority. Activity, on the other hand, hardly involves an authoritative command.
Another thing that sets task apart from activity is that the former does not involve movement, whereas movement is an integral part of an activity.
When talking about task, one is mainly talking about an activity that is done with a purpose, whereas when one talks about activity, one is simply talking about a work that is done with or without some purpose in mind.
One more thing that differentiates task from activity is the amount of energy and action that is involved in both the things. There is no real involvement of energy and action in a task. However, activity requires a significant amount of both energy and action, as that is what allows an activity to be performed.
Task and activity also differ from each other on how they are related to the actual activity. While the former basically refers to the completion of an activity, the latter refers to the movements and actions that are done to complete an activity.