The unit for measuring project's work

The answer is Time!

Yes, it is at the first sight a bit surprising since time is used also for scheduling. However, when we mention scheduling, we are more concerned about the task duration with a specific number of resources assigned to complete it.

Take care that!
A specified task duration assigned to a single resource can't be directly halved by assigning it to 2 resources:

This might be an over simplification, as usually this involves at least some interaction between the resources which can add to the communication overhead work to be performed. 

Difference between Start\Finish times, Baseline Start\Finish times, and Actual Start\Finish times for project tasks.

Baseline Start\Finish times: Those are the planned times entered during planning to specify the planned start\finish times for each task. Once the plan enters execution, this should not change; however, this remains as a baseline (reference) for comparison with the actual Task Start\Finish times for each task for the purposes of monitoring or evaluation.

Actual Start\Finish times: Those are the actual times periodically entered for each task at the head of its execution and following its completion to specify the actual duration for each task.

Start\Finish times: Those are the forecast-ed times periodically updated for planned tasks to reflect any new inputs which are excepted to change the schedule and which were not foreseen at initial project planning.