[mattao]

As far as I know:

effort is the working time necessary to do something:

Exemple: to draw this house plans it will take 40 hours work.

Duration is the time it will take.

If two people make their efforts to do this task it will take them 20 hours ( duration ) to do this 40 hours ( effort ) task.

(You will note that two people working together on a task rarely succeed in dividing is execution time by two but....)

Hope I helped.