Final answer:
A lag refers to the delay caused in the next activity due to waiting time required for the earlier activity, while fast tracking involves starting an activity before another is finished.
Step-by-step explanation:
The delay caused in the next activity because of the waiting time required to complete the earlier activity is called a lag. On the other hand, starting an activity before another activity is completed is called fast tracking.
For example, in a construction project, if the foundation needs to be completed before starting the framing work, any delay in completing the foundation will cause a lag and delay the overall project. On the other hand, if the framing work starts before the foundation is completed, it is considered fast tracking.