What is the difference between call as a task type and calls as an activity type?

What is the difference between call as a task type and calls as an activity type?

I'm getting very confused as I try to understand this point.

I can see that I can create an activity type called "call"

And that I can create an activity type called "task"

And that there exists a task type called "call"

What is the difference between the two forms of call? When would I use one, and when the other?