A lead time means the successor task will occur earlier in the schedule than
directed by the link itself, a lag time means it will occur later.
Examples - we are moving our offices and we have 10 offices to move. We
have to pack the office furnishings, computer, etc before we can load them
on the truck. Packing is the predecessor, loading is the successor, linked
finish to start. But if have enough people we don't have to pack all 10
offices before we beging loading the truck. We can bring our packing crew
in at 8 and they box up the first office. At 9 they move on to the 2nd
office and we bring our loaders in to take the first office's boxes to the
truck while the packers are in office number 2. At 10 the packers are
finished in 2 and move on to 3 while the loaders load 2's boxes on the
truck, etc, etc. The Project plan would show Packing linked to Loading FS
with a 90% lead time, ie, loading can begin when we are 10% into the overall
packing task.
The truck will take 2 weeks to get to our new location. It's being done by
an outside trucking firm that's been contracted to carry the shipment and so
we choose not to include the work of driving the truck as a task in our
project. The finish of loading at the origin links directly to the start of
unloading at the destination. The two weeks it'll take the shipment to get
there is shown as a lag time in the link btween loading and unloading.
Likewise lag can be used to show waiting for an approvals, for surveys to
come back in the mail, any delays between tasks that are "idle time" with
respect to resources actually doing some physical acitivity creating project
deliverables.
Hope this hlps.
